Is there a version of JavaScript s String indexOf that allows for regular expressions

Question

In javascript  is there an equivalent of String indexOf   that takes a regular expression instead of a string for the first first parameter while still allowing a second parameter    I need to do something like   str indexOf   abc     i     and  str lastIndexOf   abc     i     While String search   takes a regexp as a parameter it does not allow me to specify a second argument   Edit  This turned out to be harder than I originally thought so I wrote a small test function to test all the provided solutions    it assumes regexIndexOf and regexLastIndexOf have been added to the String object   function test  str        var i   str length  2      while  i--            if  str indexOf  a  i     str regexIndexOf  a  i                alert    failed regexIndexOf     str i   str indexOf  a  i    str regexIndexOf  a  i              if  str lastIndexOf  a  i     str regexLastIndexOf  a  i                 alert    failed regexLastIndexOf     str i str lastIndexOf  a  i    str regexLastIndexOf  a  i                and I am testing as follow to make sure that at least for one character regexp  the result is the same as if we used indexOf    Look for the a among the xes test  xxx    test  axx    test  xax    test  xxa    test  axa    test  xaa    test  aax    test  aaa

User · Answer

Based on BaileyP s answer  The main difference is that these methods return -1 if the pattern can t be matched   Edit  Thanks to Jason Bunting s answer I got an idea  Why not modify the  lastIndex property of the regex  Though this will only work for patterns with the global flag   g    Edit  Updated to pass the test-cases   String prototype regexIndexOf   function re  startPos        startPos   startPos    0       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              re lastIndex   startPos      var match   re exec this        if  match  return match index      else return -1     String prototype regexLastIndexOf   function re  startPos        startPos   startPos     undefined   this length   startPos       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              var lastSuccess   -1      for  var pos   0  pos  lt   startPos  pos              re lastIndex   pos           var match   re exec this           if   match  break           pos   match index          if  pos  lt   startPos  lastSuccess   pos             return lastSuccess

User · Answer

I used  String prototype match regex  which returns a string array of all found matches of the given regex in the string  more info see here      x000D   x000D  function getLastIndex text  regex  limit   text length    x000D    const matches   text match regex   x000D   x000D       no matches found x000D    if   matches    x000D      return -1  x000D      x000D   x000D       matches found but first index greater than limit x000D    if  text indexOf matches 0    matches 0  length   gt  limit    x000D      return -1  x000D      x000D   x000D       reduce index until smaller than limit x000D    let i   matches length - 1  x000D    let index   text lastIndexOf matches i    x000D    while  index  gt  limit  amp  amp  i  gt   0    x000D      i--  x000D      index   text lastIndexOf matches i    x000D      x000D    return index  gt  limit   -1   index  x000D    x000D   x000D     expect -1 as first index     14 x000D  console log getLastIndex  First Sentence  Last Sentence  Unfinished        g  10    x000D   x000D     expect 29 x000D  console log getLastIndex  First Sentence  Last Sentence  Unfinished        g    x000D   x000D   x000D

User · Answer

Instances of the String constructor have a  search   method which accepts a RegExp and returns the index of the first match   To start the search from a particular position  faking the second parameter of  indexOf    you can slice off the first i characters   str slice i  search  re     But this will get the index in the shorter string  after the first part was sliced off  so you ll want to then add the length of the chopped off part  i  to the returned index if it wasn t -1  This will give you the index in the original string   function regexIndexOf text  re  i        var indexInSuffix   text slice i  search re       return indexInSuffix  lt  0   indexInSuffix   indexInSuffix   i

User · Answer

There are still no native methods that perform the requested task   Here is the code that I am using  It mimics the behaviour of String prototype indexOf and String prototype lastIndexOf methods but they also accept a RegExp as the search argument in addition to a string representing the value to search for   Yes it is quite long as an answer goes as it tries to follow current standards as close as possible and of course contains a reasonable amount of JSDOC comments  However  once minified  the code is only 2 27k and once gzipped for transmission it is only 1023 bytes   The 2 methods that this adds to String prototype  using Object defineProperty where available  are    searchOf searchLastOf   It passes all the tests that the OP posted and additionally I have tested the routines quite thoroughly in my daily usage  and have attempted to be sure that they work across multiple environments  but feedback issues are always welcome    x000D   x000D    jslint maxlen 80  browser true    x000D   x000D     x000D     Properties used by searchOf and searchLastOf implementation  x000D      x000D   x000D    property x000D      MAX SAFE INTEGER  abs  add  apply  call  configurable  defineProperty  x000D      enumerable  exec  floor  global  hasOwnProperty  ignoreCase  index  x000D      lastIndex  lastIndexOf  length  max  min  multiline  pow  prototype  x000D      remove  replace  searchLastOf  searchOf  source  toString  value  writable x000D     x000D   x000D     x000D     Properties used in the testing of searchOf and searchLastOf implimentation  x000D      x000D   x000D    property x000D      appendChild  createTextNode  getElementById  indexOf  lastIndexOf  length  x000D      searchLastOf  searchOf  unshift x000D     x000D   x000D   function      x000D       use strict   x000D   x000D      var MAX SAFE INTEGER   Number MAX SAFE INTEGER    Math pow 2  53  - 1  x000D          getNativeFlags   new RegExp       a-z        i    x000D          clipDups   new RegExp      s  S        s  S    1     g    x000D          pToString   Object prototype toString  x000D          pHasOwn   Object prototype hasOwnProperty  x000D          stringTagRegExp  x000D   x000D          x000D         Defines a new property directly on an object  or modifies an existing x000D         property on an object  and returns the object  x000D         x000D          private x000D          function x000D          param  Object  object x000D          param  string  property x000D          param  Object  descriptor x000D          returns  Object  x000D          see https   goo gl CZnEqg x000D          x000D      function  defineProperty object  property  descriptor    x000D          if  Object defineProperty    x000D              Object defineProperty object  property  descriptor   x000D            else   x000D              object property    descriptor value  x000D            x000D   x000D          return object  x000D        x000D   x000D          x000D         Returns true if the operands are strictly equal with no type conversion  x000D         x000D          private x000D          function x000D          param     a x000D          param     b x000D          returns  boolean  x000D          see http   www ecma-international org ecma-262 5 1  sec-11 9 4 x000D          x000D      function  strictEqual a  b    x000D          return a     b  x000D        x000D   x000D          x000D         Returns true if the operand inputArg is undefined  x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  boolean  x000D          x000D      function  isUndefined inputArg    x000D          return  strictEqual typeof inputArg   undefined    x000D        x000D   x000D          x000D         Provides a string representation of the supplied object in the form x000D           object type    where type is the object type  x000D         x000D          private x000D          function x000D          param     inputArg The object for which a class string represntation x000D                             is required  x000D          returns  string  A string value of the form   object type    x000D          see http   www ecma-international org ecma-262 5 1  sec-15 2 4 2 x000D          x000D      function  toStringTag inputArg    x000D          var val  x000D          if  inputArg     null    x000D              val     object Null    x000D            else if   isUndefined inputArg     x000D              val     object Undefined    x000D            else   x000D              val   pToString call inputArg   x000D            x000D   x000D          return val  x000D        x000D   x000D          x000D         The string tag representation of a RegExp object  x000D         x000D          private x000D          type  string  x000D          x000D      stringTagRegExp    toStringTag getNativeFlags   x000D   x000D          x000D         Returns true if the operand inputArg is a RegExp  x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  boolean  x000D          x000D      function  isRegExp inputArg    x000D          return  toStringTag inputArg      stringTagRegExp  amp  amp  x000D                  pHasOwn call inputArg   ignoreCase    amp  amp  x000D                  typeof inputArg ignoreCase      boolean   amp  amp  x000D                  pHasOwn call inputArg   global    amp  amp  x000D                  typeof inputArg global      boolean   amp  amp  x000D                  pHasOwn call inputArg   multiline    amp  amp  x000D                  typeof inputArg multiline      boolean   amp  amp  x000D                  pHasOwn call inputArg   source    amp  amp  x000D                  typeof inputArg source      string   x000D        x000D   x000D          x000D         The abstract operation throws an error if its argument is a value that x000D         cannot be converted to an Object  otherwise returns the argument  x000D         x000D          private x000D          function x000D          param     inputArg The object to be tested  x000D          throws  TypeError  If inputArg is null or undefined  x000D          returns     The inputArg if coercible  x000D          see https   goo gl 5GcmVq x000D          x000D      function  requireObjectCoercible inputArg    x000D          var errStr  x000D   x000D          if  inputArg     null     isUndefined inputArg     x000D              errStr    Cannot convert argument to object      inputArg  x000D              throw new TypeError errStr   x000D            x000D   x000D          return inputArg  x000D        x000D   x000D          x000D         The abstract operation converts its argument to a value of type string x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  string  x000D          see https   people mozilla org  jorendorff es6-draft html sec-tostring x000D          x000D      function  toString inputArg    x000D          var type  x000D              val  x000D   x000D          if  inputArg     null    x000D              val    null   x000D            else   x000D              type   typeof inputArg  x000D              if  type      string     x000D                  val   inputArg  x000D                else if  type      undefined     x000D                  val   type  x000D                else   x000D                  if  type      symbol     x000D                      throw new TypeError  Cannot convert symbol to string    x000D                    x000D   x000D                  val   String inputArg   x000D                x000D            x000D   x000D          return val  x000D        x000D   x000D          x000D         Returns a string only if the arguments is coercible otherwise throws an x000D         error  x000D         x000D          private x000D          function x000D          param     inputArg x000D          throws  TypeError  If inputArg is null or undefined  x000D          returns  string  x000D          x000D      function  onlyCoercibleToString inputArg    x000D          return  toString  requireObjectCoercible inputArg    x000D        x000D   x000D          x000D         The function evaluates the passed value and converts it to an integer  x000D         x000D          private x000D          function x000D          param     inputArg The object to be converted to an integer  x000D          returns  number  If the target value is NaN  null or undefined  0 is x000D                           returned  If the target value is false  0 is returned x000D                           and if true  1 is returned  x000D          see http   www ecma-international org ecma-262 5 1  sec-9 4 x000D          x000D      function  toInteger inputArg    x000D          var number    inputArg  x000D              val   0  x000D   x000D          if   strictEqual number  number     x000D              if   number    number     Infinity    number     -Infinity    x000D                  val   number  x000D                else   x000D                  val    number  gt  0    -1    Math floor Math abs number    x000D                x000D            x000D   x000D          return val  x000D        x000D   x000D          x000D         Copies a regex object  Allows adding and removing native flags while x000D         copying the regex  x000D         x000D          private x000D          function x000D          param  RegExp  regex Regex to copy  x000D          param  Object   options  Allows specifying native flags to add or x000D                                   remove while copying the regex  x000D          returns  RegExp  Copy of the provided regex  possibly with modified x000D                           flags  x000D          x000D      function  copyRegExp regex  options    x000D          var flags  x000D              opts  x000D              rx  x000D   x000D          if  options     null  amp  amp  typeof options      object     x000D              opts   options  x000D            else   x000D              opts       x000D            x000D   x000D             Get native flags in use x000D          flags   getNativeFlags exec  toString regex   1   x000D          flags    onlyCoercibleToString flags   x000D          if  opts add    x000D              flags    opts add  x000D              flags   flags replace clipDups       x000D            x000D   x000D          if  opts remove    x000D                 Would need to escape  options remove  if this was public x000D              rx   new RegExp       opts remove          g    x000D              flags   flags replace rx       x000D            x000D   x000D          return new RegExp regex source  flags   x000D        x000D   x000D          x000D         The abstract operation ToLength converts its argument to an integer x000D         suitable for use as the length of an array-like object  x000D         x000D          private x000D          function x000D          param     inputArg The object to be converted to a length  x000D          returns  number  If len  lt    0 then  0 else if len is  INFINITY then x000D                           2 53-1 else min len  2 53-1   x000D          see https   people mozilla org  jorendorff es6-draft html sec-tolength x000D          x000D      function  toLength inputArg    x000D          return Math min Math max  toInteger inputArg   0   MAX SAFE INTEGER   x000D        x000D   x000D          x000D         Copies a regex object so that it is suitable for use with searchOf and x000D         searchLastOf methods  x000D         x000D          private x000D          function x000D          param  RegExp  regex Regex to copy  x000D          returns  RegExp  x000D          x000D      function  toSearchRegExp regex    x000D          return  copyRegExp regex    x000D              add   g   x000D              remove   y  x000D              x000D        x000D   x000D          x000D         Returns true if the operand inputArg is a member of one of the types x000D         Undefined  Null  Boolean  Number  Symbol  or String  x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  boolean  x000D          see https   goo gl W68ywJ x000D          see https   goo gl ev7881 x000D          x000D      function  isPrimitive inputArg    x000D          var type   typeof inputArg  x000D   x000D          return type      undefined     x000D                  inputArg     null    x000D                  type      boolean     x000D                  type      string     x000D                  type      number     x000D                  type      symbol   x000D        x000D   x000D          x000D         The abstract operation converts its argument to a value of type Object x000D         but fixes some environment bugs  x000D         x000D          private x000D          function x000D          param     inputArg The argument to be converted to an object  x000D          throws  TypeError  If inputArg is not coercible to an object  x000D          returns  Object  Value of inputArg as type Object  x000D          see http   www ecma-international org ecma-262 5 1  sec-9 9 x000D          x000D      function  toObject inputArg    x000D          var object  x000D   x000D          if   isPrimitive  requireObjectCoercible inputArg      x000D              object   Object inputArg   x000D            else   x000D              object   inputArg  x000D            x000D   x000D          return object  x000D        x000D   x000D          x000D         Converts a single argument that is an array-like object or list  eg  x000D         arguments  NodeList  DOMTokenList  used by classList   NamedNodeMap x000D          used by attributes property   into a new Array   and returns it  x000D         This is a partial implementation of the ES6 Array from x000D         x000D          private x000D          function x000D          param  Object  arrayLike x000D          returns  Array  x000D          x000D      function  toArray arrayLike    x000D          var object    toObject arrayLike   x000D              length    toLength object length   x000D              array       x000D              index   0  x000D   x000D          array length   length  x000D          while  index  lt  length    x000D              array index    object index   x000D              index    1  x000D            x000D   x000D          return array  x000D        x000D   x000D      if   String prototype searchOf    x000D              x000D             This method returns the index within the calling String object of x000D             the first occurrence of the specified value  starting the search at x000D             fromIndex  Returns -1 if the value is not found  x000D             x000D              function x000D              this  string  x000D              param  RegExp string  regex A regular expression object or a String  x000D                                          Anything else is implicitly converted to x000D                                          a String  x000D              param  Number   fromIndex  The location within the calling string x000D                                         to start the search from  It can be any x000D                                         integer  The default value is 0  If x000D                                         fromIndex  lt  0 the entire string is x000D                                         searched  same as passing 0   If x000D                                         fromIndex  gt   str length  the method will x000D                                         return -1 unless searchValue is an empty x000D                                         string in which case str length is x000D                                         returned  x000D              returns  Number  If successful  returns the index of the first x000D                               match of the regular expression inside the x000D                               string  Otherwise  it returns -1  x000D              x000D           defineProperty String prototype   searchOf     x000D              enumerable  false  x000D              configurable  true  x000D              writable  true  x000D              value  function  regex    x000D                  var str    onlyCoercibleToString this   x000D                      args    toArray arguments   x000D                      result   -1  x000D                      fromIndex  x000D                      match  x000D                      rx  x000D   x000D                  if    isRegExp regex     x000D                      return String prototype indexOf apply str  args   x000D                    x000D   x000D                  if   toLength args length   gt  1    x000D                      fromIndex    args 1   x000D                      if  fromIndex  lt  0    x000D                          fromIndex   0  x000D                        x000D                    else   x000D                      fromIndex   0  x000D                    x000D   x000D                  if  fromIndex  gt    toLength str length     x000D                      return result  x000D                    x000D   x000D                  rx    toSearchRegExp regex   x000D                  rx lastIndex   fromIndex  x000D                  match   rx exec str   x000D                  if  match    x000D                      result    match index  x000D                    x000D   x000D                  return result  x000D                x000D              x000D        x000D   x000D      if   String prototype searchLastOf    x000D              x000D             This method returns the index within the calling String object of x000D             the last occurrence of the specified value  or -1 if not found  x000D             The calling string is searched backward  starting at fromIndex  x000D             x000D              function x000D              this  string  x000D              param  RegExp string  regex A regular expression object or a String  x000D                                          Anything else is implicitly converted to x000D                                          a String  x000D              param  Number   fromIndex  Optional  The location within the x000D                                         calling string to start the search at  x000D                                         indexed from left to right  It can be x000D                                         any integer  The default value is x000D                                         str length  If it is negative  it is x000D                                         treated as 0  If fromIndex  gt  str length  x000D                                         fromIndex is treated as str length  x000D              returns  Number  If successful  returns the index of the first x000D                               match of the regular expression inside the x000D                               string  Otherwise  it returns -1  x000D              x000D           defineProperty String prototype   searchLastOf     x000D              enumerable  false  x000D              configurable  true  x000D              writable  true  x000D              value  function  regex    x000D                  var str    onlyCoercibleToString this   x000D                      args    toArray arguments   x000D                      result   -1  x000D                      fromIndex  x000D                      length  x000D                      match  x000D                      pos  x000D                      rx  x000D   x000D                  if    isRegExp regex     x000D                      return String prototype lastIndexOf apply str  args   x000D                    x000D   x000D                  length    toLength str length   x000D                  if    strictEqual args 1   args 1      x000D                      fromIndex   length  x000D                    else   x000D                      if   toLength args length   gt  1    x000D                          fromIndex    toInteger args 1    x000D                        else   x000D                          fromIndex   length - 1  x000D                        x000D                    x000D   x000D                  if  fromIndex  gt   0    x000D                      fromIndex   Math min fromIndex  length - 1   x000D                    else   x000D                      fromIndex   length - Math abs fromIndex   x000D                    x000D   x000D                  pos   0  x000D                  rx    toSearchRegExp regex   x000D                  while  pos  lt   fromIndex    x000D                      rx lastIndex   pos  x000D                      match   rx exec str   x000D                      if   match    x000D                          break  x000D                        x000D   x000D                      pos    match index  x000D                      if  pos  lt   fromIndex    x000D                          result   pos  x000D                        x000D   x000D                      pos    1  x000D                    x000D   x000D                  return result  x000D                x000D              x000D        x000D        x000D   x000D   function      x000D       use strict   x000D   x000D         x000D         testing as follow to make sure that at least for one character regexp  x000D         the result is the same as if we used indexOf x000D          x000D   x000D      var pre   document getElementById  out    x000D   x000D      function log result    x000D          pre appendChild document createTextNode result     n     x000D        x000D   x000D      function test str    x000D          var i   str length   2  x000D              r  x000D              a  x000D              b  x000D   x000D          while  i    x000D              a   str indexOf  a   i   x000D              b   str searchOf  a   i   x000D              r     Failed    searchOf   str  i  a  b   x000D              if  a     b    x000D                  r 0     Passed   x000D                x000D   x000D              log r   x000D              a   str lastIndexOf  a   i   x000D              b   str searchLastOf  a   i   x000D              r     Failed    searchLastOf   str  i  a  b   x000D              if  a     b    x000D                  r 0     Passed   x000D                x000D   x000D              log r   x000D              i -  1  x000D            x000D        x000D   x000D         x000D         Look for the a among the xes x000D          x000D   x000D      test  xxx    x000D      test  axx    x000D      test  xax    x000D      test  xxa    x000D      test  axa    x000D      test  xaa    x000D      test  aax    x000D      test  aaa    x000D        x000D   lt pre id  out  gt  lt  pre gt  x000D   x000D   x000D

User · Answer

Jason Bunting s last index of does not work  Mine is not optimal  but it works     Jason Bunting s String prototype regexIndexOf   function regex  startpos    var indexOf   this substring startpos    0  search regex   return  indexOf  gt   0     indexOf    startpos    0     indexOf     String prototype regexLastIndexOf   function regex  startpos    var lastIndex   -1  var index   this regexIndexOf  regex    startpos   startpos     undefined   this length   startpos   while   index  gt   0  amp  amp  index  lt  startpos         lastIndex   index      index   this regexIndexOf  regex  index   1      return lastIndex

User · Answer

After having all the proposed solutions fail my tests one way or the other   edit  some were updated to pass the tests after I wrote this  I found the mozilla implementation for Array indexOf and Array lastIndexOf  I used those to implement my version of String prototype regexIndexOf and String prototype regexLastIndexOf as follows   String prototype regexIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1      0      from    from  lt  0    Math ceil from    Math floor from       if  from  lt  0        from    len       for    from  lt  len  from            if  from in arr  amp  amp  elt exec arr from              return from            return -1      String prototype regexLastIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1        if  isNaN from           from   len - 1        else         from    from  lt  0    Math ceil from    Math floor from         if  from  lt  0          from    len        else if  from  gt   len          from   len - 1             for    from  gt  -1  from--          if  from in arr  amp  amp  elt exec arr from             return from            return -1         They seem to pass the test functions I provided in the question   Obviously they only work if the regular expression matches one character but that is enough for my purpose since I will be using it for things like    abc      s    W    D    I will keep monitoring the question in case someone provides a better faster cleaner more generic implementation that works on any regular expression

User · Answer

It does not natively  but you certainly can add this functionality   lt script type  text javascript  gt   String prototype regexIndexOf   function  pattern  startIndex         startIndex   startIndex    0      var searchResult   this substr  startIndex   search  pattern        return   -1     searchResult     -1   searchResult   startIndex     String prototype regexLastIndexOf   function  pattern  startIndex         startIndex   startIndex     undefined   this length   startIndex      var searchResult   this substr  0  startIndex   reverse   regexIndexOf  pattern  0        return   -1     searchResult     -1   this length -   searchResult     String prototype reverse   function         return this split     reverse   join            Indexes 0123456789 var str    caabbccdda    alert            str regexIndexOf    cd    4           str regexLastIndexOf    cd    4           str regexIndexOf    yz    4           str regexLastIndexOf    yz    4           str lastIndexOf   d   4           str regexLastIndexOf   d   4           str lastIndexOf   d            str regexLastIndexOf   d               lt  script gt    I didn t fully test these methods  but they seem to work so far

User · Answer

Jason Bunting s last index of does not work  Mine is not optimal  but it works     Jason Bunting s String prototype regexIndexOf   function regex  startpos    var indexOf   this substring startpos    0  search regex   return  indexOf  gt   0     indexOf    startpos    0     indexOf     String prototype regexLastIndexOf   function regex  startpos    var lastIndex   -1  var index   this regexIndexOf  regex    startpos   startpos     undefined   this length   startpos   while   index  gt   0  amp  amp  index  lt  startpos         lastIndex   index      index   this regexIndexOf  regex  index   1      return lastIndex

User · Answer

Well  as you are just looking to match the position of a character   regex is possibly overkill    I presume all you want is  instead of  find first of these this character    just find first of these characters    This of course is the simple answer  but does what your question sets out to do  albeit without the regex part   because you didn t clarify why specifically it had to be a regex    function mIndexOf  str   chars  offset        var first    -1      for  var i   0  i  lt  chars length   i                var p   str indexOf  chars i    offset           if  p  lt  first    first     -1                      first   p                  return first     String prototype mIndexOf   function  chars  offset        return mIndexOf  this  chars  offset      I m really averse to monkey patching       mIndexOf   hello world     a   o   w    0     gt  gt  4  mIndexOf   hello world     a    0     gt  gt  -1  mIndexOf   hello world     a   o   w    4     gt  gt  4 mIndexOf   hello world     a   o   w    5     gt  gt  6 mIndexOf   hello world     a   o   w    7     gt  gt  -1  mIndexOf   hello world     a   o   w   d    7     gt  gt  10 mIndexOf   hello world     a   o   w   d    10     gt  gt  10 mIndexOf   hello world     a   o   w   d    11     gt  gt  -1

User · Answer

Well  as you are just looking to match the position of a character   regex is possibly overkill    I presume all you want is  instead of  find first of these this character    just find first of these characters    This of course is the simple answer  but does what your question sets out to do  albeit without the regex part   because you didn t clarify why specifically it had to be a regex    function mIndexOf  str   chars  offset        var first    -1      for  var i   0  i  lt  chars length   i                var p   str indexOf  chars i    offset           if  p  lt  first    first     -1                      first   p                  return first     String prototype mIndexOf   function  chars  offset        return mIndexOf  this  chars  offset      I m really averse to monkey patching       mIndexOf   hello world     a   o   w    0     gt  gt  4  mIndexOf   hello world     a    0     gt  gt  -1  mIndexOf   hello world     a   o   w    4     gt  gt  4 mIndexOf   hello world     a   o   w    5     gt  gt  6 mIndexOf   hello world     a   o   w    7     gt  gt  -1  mIndexOf   hello world     a   o   w   d    7     gt  gt  10 mIndexOf   hello world     a   o   w   d    10     gt  gt  10 mIndexOf   hello world     a   o   w   d    11     gt  gt  -1

User · Answer

I used  String prototype match regex  which returns a string array of all found matches of the given regex in the string  more info see here      x000D   x000D  function getLastIndex text  regex  limit   text length    x000D    const matches   text match regex   x000D   x000D       no matches found x000D    if   matches    x000D      return -1  x000D      x000D   x000D       matches found but first index greater than limit x000D    if  text indexOf matches 0    matches 0  length   gt  limit    x000D      return -1  x000D      x000D   x000D       reduce index until smaller than limit x000D    let i   matches length - 1  x000D    let index   text lastIndexOf matches i    x000D    while  index  gt  limit  amp  amp  i  gt   0    x000D      i--  x000D      index   text lastIndexOf matches i    x000D      x000D    return index  gt  limit   -1   index  x000D    x000D   x000D     expect -1 as first index     14 x000D  console log getLastIndex  First Sentence  Last Sentence  Unfinished        g  10    x000D   x000D     expect 29 x000D  console log getLastIndex  First Sentence  Last Sentence  Unfinished        g    x000D   x000D   x000D

User · Answer

You could use substr   str substr i  match   abc

User · Answer

Use   str search regex    See the documentation here

User · Answer

RexExp instances have a lastIndex property already  if they are global  and so what I m doing is copying the regular expression  modifying it slightly to suit our purposes  exec-ing it on the string and looking at the lastIndex   This will inevitably be faster than looping on the string    You have enough examples of how to put this onto the string prototype  right    function reIndexOf reIn  str  startIndex        var re   new RegExp reIn source   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      function reLastIndexOf reIn  str  startIndex        var src         test reIn source   amp  amp           test reIn source    reIn source   reIn source          S  s      reIn source            var re   new RegExp src   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      reIndexOf   abc     tommy can eat        Returns 6 reIndexOf   abc     tommy can eat   8       Returns 11 reLastIndexOf   abc     tommy can eat       Returns 11   You could also prototype the functions onto the RegExp object   RegExp prototype indexOf   function str  startIndex        var re   new RegExp this source   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      RegExp prototype lastIndexOf   function str  startIndex        var src         test this source   amp  amp           test this source    this source   this source          S  s      this source            var re   new RegExp src   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length         abc   indexOf  tommy can eat        Returns 6   abc   indexOf  tommy can eat   8       Returns 11   abc   lastIndexOf  tommy can eat       Returns 11   A quick explanation of how I am modifying the RegExp   For indexOf I just have to ensure that the global flag is set   For lastIndexOf of I am using a negative look-ahead to find the last occurrence unless the RegExp was already matching at the end of the string

User · Answer

Based on BaileyP s answer  The main difference is that these methods return -1 if the pattern can t be matched   Edit  Thanks to Jason Bunting s answer I got an idea  Why not modify the  lastIndex property of the regex  Though this will only work for patterns with the global flag   g    Edit  Updated to pass the test-cases   String prototype regexIndexOf   function re  startPos        startPos   startPos    0       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              re lastIndex   startPos      var match   re exec this        if  match  return match index      else return -1     String prototype regexLastIndexOf   function re  startPos        startPos   startPos     undefined   this length   startPos       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              var lastSuccess   -1      for  var pos   0  pos  lt   startPos  pos              re lastIndex   pos           var match   re exec this           if   match  break           pos   match index          if  pos  lt   startPos  lastSuccess   pos             return lastSuccess

User · Answer

It does not natively  but you certainly can add this functionality   lt script type  text javascript  gt   String prototype regexIndexOf   function  pattern  startIndex         startIndex   startIndex    0      var searchResult   this substr  startIndex   search  pattern        return   -1     searchResult     -1   searchResult   startIndex     String prototype regexLastIndexOf   function  pattern  startIndex         startIndex   startIndex     undefined   this length   startIndex      var searchResult   this substr  0  startIndex   reverse   regexIndexOf  pattern  0        return   -1     searchResult     -1   this length -   searchResult     String prototype reverse   function         return this split     reverse   join            Indexes 0123456789 var str    caabbccdda    alert            str regexIndexOf    cd    4           str regexLastIndexOf    cd    4           str regexIndexOf    yz    4           str regexLastIndexOf    yz    4           str lastIndexOf   d   4           str regexLastIndexOf   d   4           str lastIndexOf   d            str regexLastIndexOf   d               lt  script gt    I didn t fully test these methods  but they seem to work so far

User · Answer

var mystring    abc ab a   var re    new RegExp  ab       any regex here  if   re exec mystring     null        alert  matches       true in this case     Use standard regular expressions   var re    new RegExp   ab        At front var re    new RegExp  ab         At end var re    new RegExp  ab c d         abc or abd

User · Answer

I have a short version for you  It works well for me   var match        str match   abc  gi   var firstIndex   str indexOf match 0    var lastIndex    str lastIndexOf match match length-1      And if you want a prototype version   String prototype indexOfRegex   function regex     var match   this match regex     return match   this indexOf match 0     -1     String prototype lastIndexOfRegex   function regex     var match   this match regex     return match   this lastIndexOf match match length-1     -1      EDIT   if you want to add support for fromIndex  String prototype indexOfRegex   function regex  fromIndex     var str   fromIndex   this substring fromIndex    this    var match   str match regex     return match   str indexOf match 0     fromIndex   -1     String prototype lastIndexOfRegex   function regex  fromIndex     var str   fromIndex   this substring 0  fromIndex    this    var match   str match regex     return match   str lastIndexOf match match length-1     -1      To use it  as simple as this   var firstIndex   str indexOfRegex   abc  gi   var lastIndex    str lastIndexOfRegex   abc  gi

User · Answer

It does not natively  but you certainly can add this functionality   lt script type  text javascript  gt   String prototype regexIndexOf   function  pattern  startIndex         startIndex   startIndex    0      var searchResult   this substr  startIndex   search  pattern        return   -1     searchResult     -1   searchResult   startIndex     String prototype regexLastIndexOf   function  pattern  startIndex         startIndex   startIndex     undefined   this length   startIndex      var searchResult   this substr  0  startIndex   reverse   regexIndexOf  pattern  0        return   -1     searchResult     -1   this length -   searchResult     String prototype reverse   function         return this split     reverse   join            Indexes 0123456789 var str    caabbccdda    alert            str regexIndexOf    cd    4           str regexLastIndexOf    cd    4           str regexIndexOf    yz    4           str regexLastIndexOf    yz    4           str lastIndexOf   d   4           str regexLastIndexOf   d   4           str lastIndexOf   d            str regexLastIndexOf   d               lt  script gt    I didn t fully test these methods  but they seem to work so far

User · Answer

After having all the proposed solutions fail my tests one way or the other   edit  some were updated to pass the tests after I wrote this  I found the mozilla implementation for Array indexOf and Array lastIndexOf  I used those to implement my version of String prototype regexIndexOf and String prototype regexLastIndexOf as follows   String prototype regexIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1      0      from    from  lt  0    Math ceil from    Math floor from       if  from  lt  0        from    len       for    from  lt  len  from            if  from in arr  amp  amp  elt exec arr from              return from            return -1      String prototype regexLastIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1        if  isNaN from           from   len - 1        else         from    from  lt  0    Math ceil from    Math floor from         if  from  lt  0          from    len        else if  from  gt   len          from   len - 1             for    from  gt  -1  from--          if  from in arr  amp  amp  elt exec arr from             return from            return -1         They seem to pass the test functions I provided in the question   Obviously they only work if the regular expression matches one character but that is enough for my purpose since I will be using it for things like    abc      s    W    D    I will keep monitoring the question in case someone provides a better faster cleaner more generic implementation that works on any regular expression

User · Answer

It does not natively  but you certainly can add this functionality   lt script type  text javascript  gt   String prototype regexIndexOf   function  pattern  startIndex         startIndex   startIndex    0      var searchResult   this substr  startIndex   search  pattern        return   -1     searchResult     -1   searchResult   startIndex     String prototype regexLastIndexOf   function  pattern  startIndex         startIndex   startIndex     undefined   this length   startIndex      var searchResult   this substr  0  startIndex   reverse   regexIndexOf  pattern  0        return   -1     searchResult     -1   this length -   searchResult     String prototype reverse   function         return this split     reverse   join            Indexes 0123456789 var str    caabbccdda    alert            str regexIndexOf    cd    4           str regexLastIndexOf    cd    4           str regexIndexOf    yz    4           str regexLastIndexOf    yz    4           str lastIndexOf   d   4           str regexLastIndexOf   d   4           str lastIndexOf   d            str regexLastIndexOf   d               lt  script gt    I didn t fully test these methods  but they seem to work so far

User · Answer

For data with sparse matches  using string search is the fastest across browsers  It re-slices a string each iteration to    function lastIndexOfSearch string  regex  index      if index     0    index       string   string slice 0  Math max 0 index      var idx    var offset   -1    while   idx   string search regex       -1        offset    idx   1      string   string slice idx   1         return offset      For dense data I made this  It s complex compared to the execute method  but for dense data  it s 2-10x faster than every other method I tried  and about 100x faster than the accepted solution  The main points are    It calls exec on the regex passed in once to verify there is a match or quit early  I do this using     in a similar method  but on IE checking with exec is dramatically faster  It constructs and caches a modified regex in the format   r       r   The new regex is executed and the results from either that exec  or the first exec  are returned   function lastIndexOfGroupSimple string  regex  index        if  index     0    index  string   string slice 0  Math max 0  index   1        regex lastIndex   0      var lastRegex  index     flags    g     regex multiline    m           regex ignoreCase    i             key   regex source         flags      match   regex exec string       if   match  return -1      if  lastIndexOfGroupSimple cache     undefined  lastIndexOfGroupSimple cache           lastRegex   lastIndexOfGroupSimple cache key       if   lastRegex          lastIndexOfGroupSimple cache key    lastRegex   new RegExp         regex source               regex source        flags       index   match index      lastRegex lastIndex   match index      return  match   lastRegex exec string     lastRegex lastIndex - match 1  length   index        jsPerf of methods  I don t understand the purpose of the tests up top  Situations that require a regex are impossible to compare against a call to indexOf  which I think is the point of making the method in the first place  To get the test to pass  it makes more sense to use  xxx    x    than adjust the way the regex iterates

User · Answer

If you are looking for a very simple lastIndex lookup with RegExp and don t care if it mimics lastIndexOf to the last detail  this may catch your attention   I simply reverse the string  and subtract the first occurence index from length - 1  It happens to pass my test  but I think there could arise a performance issue with long strings   interface String     reverse    string    lastIndex regex  RegExp   number     String prototype reverse   function this  string      return this split          reverse        join          String prototype lastIndex   function this  string  regex  RegExp      const exec   regex exec this reverse       return exec     null   -1   this length - 1 - exec index

User · Answer

There are still no native methods that perform the requested task   Here is the code that I am using  It mimics the behaviour of String prototype indexOf and String prototype lastIndexOf methods but they also accept a RegExp as the search argument in addition to a string representing the value to search for   Yes it is quite long as an answer goes as it tries to follow current standards as close as possible and of course contains a reasonable amount of JSDOC comments  However  once minified  the code is only 2 27k and once gzipped for transmission it is only 1023 bytes   The 2 methods that this adds to String prototype  using Object defineProperty where available  are    searchOf searchLastOf   It passes all the tests that the OP posted and additionally I have tested the routines quite thoroughly in my daily usage  and have attempted to be sure that they work across multiple environments  but feedback issues are always welcome    x000D   x000D    jslint maxlen 80  browser true    x000D   x000D     x000D     Properties used by searchOf and searchLastOf implementation  x000D      x000D   x000D    property x000D      MAX SAFE INTEGER  abs  add  apply  call  configurable  defineProperty  x000D      enumerable  exec  floor  global  hasOwnProperty  ignoreCase  index  x000D      lastIndex  lastIndexOf  length  max  min  multiline  pow  prototype  x000D      remove  replace  searchLastOf  searchOf  source  toString  value  writable x000D     x000D   x000D     x000D     Properties used in the testing of searchOf and searchLastOf implimentation  x000D      x000D   x000D    property x000D      appendChild  createTextNode  getElementById  indexOf  lastIndexOf  length  x000D      searchLastOf  searchOf  unshift x000D     x000D   x000D   function      x000D       use strict   x000D   x000D      var MAX SAFE INTEGER   Number MAX SAFE INTEGER    Math pow 2  53  - 1  x000D          getNativeFlags   new RegExp       a-z        i    x000D          clipDups   new RegExp      s  S        s  S    1     g    x000D          pToString   Object prototype toString  x000D          pHasOwn   Object prototype hasOwnProperty  x000D          stringTagRegExp  x000D   x000D          x000D         Defines a new property directly on an object  or modifies an existing x000D         property on an object  and returns the object  x000D         x000D          private x000D          function x000D          param  Object  object x000D          param  string  property x000D          param  Object  descriptor x000D          returns  Object  x000D          see https   goo gl CZnEqg x000D          x000D      function  defineProperty object  property  descriptor    x000D          if  Object defineProperty    x000D              Object defineProperty object  property  descriptor   x000D            else   x000D              object property    descriptor value  x000D            x000D   x000D          return object  x000D        x000D   x000D          x000D         Returns true if the operands are strictly equal with no type conversion  x000D         x000D          private x000D          function x000D          param     a x000D          param     b x000D          returns  boolean  x000D          see http   www ecma-international org ecma-262 5 1  sec-11 9 4 x000D          x000D      function  strictEqual a  b    x000D          return a     b  x000D        x000D   x000D          x000D         Returns true if the operand inputArg is undefined  x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  boolean  x000D          x000D      function  isUndefined inputArg    x000D          return  strictEqual typeof inputArg   undefined    x000D        x000D   x000D          x000D         Provides a string representation of the supplied object in the form x000D           object type    where type is the object type  x000D         x000D          private x000D          function x000D          param     inputArg The object for which a class string represntation x000D                             is required  x000D          returns  string  A string value of the form   object type    x000D          see http   www ecma-international org ecma-262 5 1  sec-15 2 4 2 x000D          x000D      function  toStringTag inputArg    x000D          var val  x000D          if  inputArg     null    x000D              val     object Null    x000D            else if   isUndefined inputArg     x000D              val     object Undefined    x000D            else   x000D              val   pToString call inputArg   x000D            x000D   x000D          return val  x000D        x000D   x000D          x000D         The string tag representation of a RegExp object  x000D         x000D          private x000D          type  string  x000D          x000D      stringTagRegExp    toStringTag getNativeFlags   x000D   x000D          x000D         Returns true if the operand inputArg is a RegExp  x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  boolean  x000D          x000D      function  isRegExp inputArg    x000D          return  toStringTag inputArg      stringTagRegExp  amp  amp  x000D                  pHasOwn call inputArg   ignoreCase    amp  amp  x000D                  typeof inputArg ignoreCase      boolean   amp  amp  x000D                  pHasOwn call inputArg   global    amp  amp  x000D                  typeof inputArg global      boolean   amp  amp  x000D                  pHasOwn call inputArg   multiline    amp  amp  x000D                  typeof inputArg multiline      boolean   amp  amp  x000D                  pHasOwn call inputArg   source    amp  amp  x000D                  typeof inputArg source      string   x000D        x000D   x000D          x000D         The abstract operation throws an error if its argument is a value that x000D         cannot be converted to an Object  otherwise returns the argument  x000D         x000D          private x000D          function x000D          param     inputArg The object to be tested  x000D          throws  TypeError  If inputArg is null or undefined  x000D          returns     The inputArg if coercible  x000D          see https   goo gl 5GcmVq x000D          x000D      function  requireObjectCoercible inputArg    x000D          var errStr  x000D   x000D          if  inputArg     null     isUndefined inputArg     x000D              errStr    Cannot convert argument to object      inputArg  x000D              throw new TypeError errStr   x000D            x000D   x000D          return inputArg  x000D        x000D   x000D          x000D         The abstract operation converts its argument to a value of type string x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  string  x000D          see https   people mozilla org  jorendorff es6-draft html sec-tostring x000D          x000D      function  toString inputArg    x000D          var type  x000D              val  x000D   x000D          if  inputArg     null    x000D              val    null   x000D            else   x000D              type   typeof inputArg  x000D              if  type      string     x000D                  val   inputArg  x000D                else if  type      undefined     x000D                  val   type  x000D                else   x000D                  if  type      symbol     x000D                      throw new TypeError  Cannot convert symbol to string    x000D                    x000D   x000D                  val   String inputArg   x000D                x000D            x000D   x000D          return val  x000D        x000D   x000D          x000D         Returns a string only if the arguments is coercible otherwise throws an x000D         error  x000D         x000D          private x000D          function x000D          param     inputArg x000D          throws  TypeError  If inputArg is null or undefined  x000D          returns  string  x000D          x000D      function  onlyCoercibleToString inputArg    x000D          return  toString  requireObjectCoercible inputArg    x000D        x000D   x000D          x000D         The function evaluates the passed value and converts it to an integer  x000D         x000D          private x000D          function x000D          param     inputArg The object to be converted to an integer  x000D          returns  number  If the target value is NaN  null or undefined  0 is x000D                           returned  If the target value is false  0 is returned x000D                           and if true  1 is returned  x000D          see http   www ecma-international org ecma-262 5 1  sec-9 4 x000D          x000D      function  toInteger inputArg    x000D          var number    inputArg  x000D              val   0  x000D   x000D          if   strictEqual number  number     x000D              if   number    number     Infinity    number     -Infinity    x000D                  val   number  x000D                else   x000D                  val    number  gt  0    -1    Math floor Math abs number    x000D                x000D            x000D   x000D          return val  x000D        x000D   x000D          x000D         Copies a regex object  Allows adding and removing native flags while x000D         copying the regex  x000D         x000D          private x000D          function x000D          param  RegExp  regex Regex to copy  x000D          param  Object   options  Allows specifying native flags to add or x000D                                   remove while copying the regex  x000D          returns  RegExp  Copy of the provided regex  possibly with modified x000D                           flags  x000D          x000D      function  copyRegExp regex  options    x000D          var flags  x000D              opts  x000D              rx  x000D   x000D          if  options     null  amp  amp  typeof options      object     x000D              opts   options  x000D            else   x000D              opts       x000D            x000D   x000D             Get native flags in use x000D          flags   getNativeFlags exec  toString regex   1   x000D          flags    onlyCoercibleToString flags   x000D          if  opts add    x000D              flags    opts add  x000D              flags   flags replace clipDups       x000D            x000D   x000D          if  opts remove    x000D                 Would need to escape  options remove  if this was public x000D              rx   new RegExp       opts remove          g    x000D              flags   flags replace rx       x000D            x000D   x000D          return new RegExp regex source  flags   x000D        x000D   x000D          x000D         The abstract operation ToLength converts its argument to an integer x000D         suitable for use as the length of an array-like object  x000D         x000D          private x000D          function x000D          param     inputArg The object to be converted to a length  x000D          returns  number  If len  lt    0 then  0 else if len is  INFINITY then x000D                           2 53-1 else min len  2 53-1   x000D          see https   people mozilla org  jorendorff es6-draft html sec-tolength x000D          x000D      function  toLength inputArg    x000D          return Math min Math max  toInteger inputArg   0   MAX SAFE INTEGER   x000D        x000D   x000D          x000D         Copies a regex object so that it is suitable for use with searchOf and x000D         searchLastOf methods  x000D         x000D          private x000D          function x000D          param  RegExp  regex Regex to copy  x000D          returns  RegExp  x000D          x000D      function  toSearchRegExp regex    x000D          return  copyRegExp regex    x000D              add   g   x000D              remove   y  x000D              x000D        x000D   x000D          x000D         Returns true if the operand inputArg is a member of one of the types x000D         Undefined  Null  Boolean  Number  Symbol  or String  x000D         x000D          private x000D          function x000D          param     inputArg x000D          returns  boolean  x000D          see https   goo gl W68ywJ x000D          see https   goo gl ev7881 x000D          x000D      function  isPrimitive inputArg    x000D          var type   typeof inputArg  x000D   x000D          return type      undefined     x000D                  inputArg     null    x000D                  type      boolean     x000D                  type      string     x000D                  type      number     x000D                  type      symbol   x000D        x000D   x000D          x000D         The abstract operation converts its argument to a value of type Object x000D         but fixes some environment bugs  x000D         x000D          private x000D          function x000D          param     inputArg The argument to be converted to an object  x000D          throws  TypeError  If inputArg is not coercible to an object  x000D          returns  Object  Value of inputArg as type Object  x000D          see http   www ecma-international org ecma-262 5 1  sec-9 9 x000D          x000D      function  toObject inputArg    x000D          var object  x000D   x000D          if   isPrimitive  requireObjectCoercible inputArg      x000D              object   Object inputArg   x000D            else   x000D              object   inputArg  x000D            x000D   x000D          return object  x000D        x000D   x000D          x000D         Converts a single argument that is an array-like object or list  eg  x000D         arguments  NodeList  DOMTokenList  used by classList   NamedNodeMap x000D          used by attributes property   into a new Array   and returns it  x000D         This is a partial implementation of the ES6 Array from x000D         x000D          private x000D          function x000D          param  Object  arrayLike x000D          returns  Array  x000D          x000D      function  toArray arrayLike    x000D          var object    toObject arrayLike   x000D              length    toLength object length   x000D              array       x000D              index   0  x000D   x000D          array length   length  x000D          while  index  lt  length    x000D              array index    object index   x000D              index    1  x000D            x000D   x000D          return array  x000D        x000D   x000D      if   String prototype searchOf    x000D              x000D             This method returns the index within the calling String object of x000D             the first occurrence of the specified value  starting the search at x000D             fromIndex  Returns -1 if the value is not found  x000D             x000D              function x000D              this  string  x000D              param  RegExp string  regex A regular expression object or a String  x000D                                          Anything else is implicitly converted to x000D                                          a String  x000D              param  Number   fromIndex  The location within the calling string x000D                                         to start the search from  It can be any x000D                                         integer  The default value is 0  If x000D                                         fromIndex  lt  0 the entire string is x000D                                         searched  same as passing 0   If x000D                                         fromIndex  gt   str length  the method will x000D                                         return -1 unless searchValue is an empty x000D                                         string in which case str length is x000D                                         returned  x000D              returns  Number  If successful  returns the index of the first x000D                               match of the regular expression inside the x000D                               string  Otherwise  it returns -1  x000D              x000D           defineProperty String prototype   searchOf     x000D              enumerable  false  x000D              configurable  true  x000D              writable  true  x000D              value  function  regex    x000D                  var str    onlyCoercibleToString this   x000D                      args    toArray arguments   x000D                      result   -1  x000D                      fromIndex  x000D                      match  x000D                      rx  x000D   x000D                  if    isRegExp regex     x000D                      return String prototype indexOf apply str  args   x000D                    x000D   x000D                  if   toLength args length   gt  1    x000D                      fromIndex    args 1   x000D                      if  fromIndex  lt  0    x000D                          fromIndex   0  x000D                        x000D                    else   x000D                      fromIndex   0  x000D                    x000D   x000D                  if  fromIndex  gt    toLength str length     x000D                      return result  x000D                    x000D   x000D                  rx    toSearchRegExp regex   x000D                  rx lastIndex   fromIndex  x000D                  match   rx exec str   x000D                  if  match    x000D                      result    match index  x000D                    x000D   x000D                  return result  x000D                x000D              x000D        x000D   x000D      if   String prototype searchLastOf    x000D              x000D             This method returns the index within the calling String object of x000D             the last occurrence of the specified value  or -1 if not found  x000D             The calling string is searched backward  starting at fromIndex  x000D             x000D              function x000D              this  string  x000D              param  RegExp string  regex A regular expression object or a String  x000D                                          Anything else is implicitly converted to x000D                                          a String  x000D              param  Number   fromIndex  Optional  The location within the x000D                                         calling string to start the search at  x000D                                         indexed from left to right  It can be x000D                                         any integer  The default value is x000D                                         str length  If it is negative  it is x000D                                         treated as 0  If fromIndex  gt  str length  x000D                                         fromIndex is treated as str length  x000D              returns  Number  If successful  returns the index of the first x000D                               match of the regular expression inside the x000D                               string  Otherwise  it returns -1  x000D              x000D           defineProperty String prototype   searchLastOf     x000D              enumerable  false  x000D              configurable  true  x000D              writable  true  x000D              value  function  regex    x000D                  var str    onlyCoercibleToString this   x000D                      args    toArray arguments   x000D                      result   -1  x000D                      fromIndex  x000D                      length  x000D                      match  x000D                      pos  x000D                      rx  x000D   x000D                  if    isRegExp regex     x000D                      return String prototype lastIndexOf apply str  args   x000D                    x000D   x000D                  length    toLength str length   x000D                  if    strictEqual args 1   args 1      x000D                      fromIndex   length  x000D                    else   x000D                      if   toLength args length   gt  1    x000D                          fromIndex    toInteger args 1    x000D                        else   x000D                          fromIndex   length - 1  x000D                        x000D                    x000D   x000D                  if  fromIndex  gt   0    x000D                      fromIndex   Math min fromIndex  length - 1   x000D                    else   x000D                      fromIndex   length - Math abs fromIndex   x000D                    x000D   x000D                  pos   0  x000D                  rx    toSearchRegExp regex   x000D                  while  pos  lt   fromIndex    x000D                      rx lastIndex   pos  x000D                      match   rx exec str   x000D                      if   match    x000D                          break  x000D                        x000D   x000D                      pos    match index  x000D                      if  pos  lt   fromIndex    x000D                          result   pos  x000D                        x000D   x000D                      pos    1  x000D                    x000D   x000D                  return result  x000D                x000D              x000D        x000D        x000D   x000D   function      x000D       use strict   x000D   x000D         x000D         testing as follow to make sure that at least for one character regexp  x000D         the result is the same as if we used indexOf x000D          x000D   x000D      var pre   document getElementById  out    x000D   x000D      function log result    x000D          pre appendChild document createTextNode result     n     x000D        x000D   x000D      function test str    x000D          var i   str length   2  x000D              r  x000D              a  x000D              b  x000D   x000D          while  i    x000D              a   str indexOf  a   i   x000D              b   str searchOf  a   i   x000D              r     Failed    searchOf   str  i  a  b   x000D              if  a     b    x000D                  r 0     Passed   x000D                x000D   x000D              log r   x000D              a   str lastIndexOf  a   i   x000D              b   str searchLastOf  a   i   x000D              r     Failed    searchLastOf   str  i  a  b   x000D              if  a     b    x000D                  r 0     Passed   x000D                x000D   x000D              log r   x000D              i -  1  x000D            x000D        x000D   x000D         x000D         Look for the a among the xes x000D          x000D   x000D      test  xxx    x000D      test  axx    x000D      test  xax    x000D      test  xxa    x000D      test  axa    x000D      test  xaa    x000D      test  aax    x000D      test  aaa    x000D        x000D   lt pre id  out  gt  lt  pre gt  x000D   x000D   x000D

User · Answer

Instances of the String constructor have a  search   method which accepts a RegExp and returns the index of the first match   To start the search from a particular position  faking the second parameter of  indexOf    you can slice off the first i characters   str slice i  search  re     But this will get the index in the shorter string  after the first part was sliced off  so you ll want to then add the length of the chopped off part  i  to the returned index if it wasn t -1  This will give you the index in the original string   function regexIndexOf text  re  i        var indexInSuffix   text slice i  search re       return indexInSuffix  lt  0   indexInSuffix   indexInSuffix   i

User · Answer

I have a short version for you  It works well for me   var match        str match   abc  gi   var firstIndex   str indexOf match 0    var lastIndex    str lastIndexOf match match length-1      And if you want a prototype version   String prototype indexOfRegex   function regex     var match   this match regex     return match   this indexOf match 0     -1     String prototype lastIndexOfRegex   function regex     var match   this match regex     return match   this lastIndexOf match match length-1     -1      EDIT   if you want to add support for fromIndex  String prototype indexOfRegex   function regex  fromIndex     var str   fromIndex   this substring fromIndex    this    var match   str match regex     return match   str indexOf match 0     fromIndex   -1     String prototype lastIndexOfRegex   function regex  fromIndex     var str   fromIndex   this substring 0  fromIndex    this    var match   str match regex     return match   str lastIndexOf match match length-1     -1      To use it  as simple as this   var firstIndex   str indexOfRegex   abc  gi   var lastIndex    str lastIndexOfRegex   abc  gi

User · Answer

You could use substr   str substr i  match   abc

User · Answer

Well  as you are just looking to match the position of a character   regex is possibly overkill    I presume all you want is  instead of  find first of these this character    just find first of these characters    This of course is the simple answer  but does what your question sets out to do  albeit without the regex part   because you didn t clarify why specifically it had to be a regex    function mIndexOf  str   chars  offset        var first    -1      for  var i   0  i  lt  chars length   i                var p   str indexOf  chars i    offset           if  p  lt  first    first     -1                      first   p                  return first     String prototype mIndexOf   function  chars  offset        return mIndexOf  this  chars  offset      I m really averse to monkey patching       mIndexOf   hello world     a   o   w    0     gt  gt  4  mIndexOf   hello world     a    0     gt  gt  -1  mIndexOf   hello world     a   o   w    4     gt  gt  4 mIndexOf   hello world     a   o   w    5     gt  gt  6 mIndexOf   hello world     a   o   w    7     gt  gt  -1  mIndexOf   hello world     a   o   w   d    7     gt  gt  10 mIndexOf   hello world     a   o   w   d    10     gt  gt  10 mIndexOf   hello world     a   o   w   d    11     gt  gt  -1

User · Answer

Instances of the String constructor have a  search   method which accepts a RegExp and returns the index of the first match   To start the search from a particular position  faking the second parameter of  indexOf    you can slice off the first i characters   str slice i  search  re     But this will get the index in the shorter string  after the first part was sliced off  so you ll want to then add the length of the chopped off part  i  to the returned index if it wasn t -1  This will give you the index in the original string   function regexIndexOf text  re  i        var indexInSuffix   text slice i  search re       return indexInSuffix  lt  0   indexInSuffix   indexInSuffix   i

User · Answer

RexExp instances have a lastIndex property already  if they are global  and so what I m doing is copying the regular expression  modifying it slightly to suit our purposes  exec-ing it on the string and looking at the lastIndex   This will inevitably be faster than looping on the string    You have enough examples of how to put this onto the string prototype  right    function reIndexOf reIn  str  startIndex        var re   new RegExp reIn source   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      function reLastIndexOf reIn  str  startIndex        var src         test reIn source   amp  amp           test reIn source    reIn source   reIn source          S  s      reIn source            var re   new RegExp src   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      reIndexOf   abc     tommy can eat        Returns 6 reIndexOf   abc     tommy can eat   8       Returns 11 reLastIndexOf   abc     tommy can eat       Returns 11   You could also prototype the functions onto the RegExp object   RegExp prototype indexOf   function str  startIndex        var re   new RegExp this source   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      RegExp prototype lastIndexOf   function str  startIndex        var src         test this source   amp  amp           test this source    this source   this source          S  s      this source            var re   new RegExp src   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length         abc   indexOf  tommy can eat        Returns 6   abc   indexOf  tommy can eat   8       Returns 11   abc   lastIndexOf  tommy can eat       Returns 11   A quick explanation of how I am modifying the RegExp   For indexOf I just have to ensure that the global flag is set   For lastIndexOf of I am using a negative look-ahead to find the last occurrence unless the RegExp was already matching at the end of the string

User · Answer

var mystring    abc ab a   var re    new RegExp  ab       any regex here  if   re exec mystring     null        alert  matches       true in this case     Use standard regular expressions   var re    new RegExp   ab        At front var re    new RegExp  ab         At end var re    new RegExp  ab c d         abc or abd

User · Answer

Based on BaileyP s answer  The main difference is that these methods return -1 if the pattern can t be matched   Edit  Thanks to Jason Bunting s answer I got an idea  Why not modify the  lastIndex property of the regex  Though this will only work for patterns with the global flag   g    Edit  Updated to pass the test-cases   String prototype regexIndexOf   function re  startPos        startPos   startPos    0       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              re lastIndex   startPos      var match   re exec this        if  match  return match index      else return -1     String prototype regexLastIndexOf   function re  startPos        startPos   startPos     undefined   this length   startPos       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              var lastSuccess   -1      for  var pos   0  pos  lt   startPos  pos              re lastIndex   pos           var match   re exec this           if   match  break           pos   match index          if  pos  lt   startPos  lastSuccess   pos             return lastSuccess

User · Answer

I needed a regexIndexOf function also for an array  so I programed one myself  However I doubt  that it s optimized  but i guess it should work properly   Array prototype regexIndexOf   function  regex  startpos   0        len   this length      for x   startpos  x  lt  len  x             if typeof this x      undefined   amp  amp      this x   match regex                return x                      return -1     arr       arr push null   arr push NaN   arr 3    7  arr push  asdf    arr push  qwer    arr push 9   arr push         console log arr   arr regexIndexOf   d   4

User · Answer

RexExp instances have a lastIndex property already  if they are global  and so what I m doing is copying the regular expression  modifying it slightly to suit our purposes  exec-ing it on the string and looking at the lastIndex   This will inevitably be faster than looping on the string    You have enough examples of how to put this onto the string prototype  right    function reIndexOf reIn  str  startIndex        var re   new RegExp reIn source   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      function reLastIndexOf reIn  str  startIndex        var src         test reIn source   amp  amp           test reIn source    reIn source   reIn source          S  s      reIn source            var re   new RegExp src   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      reIndexOf   abc     tommy can eat        Returns 6 reIndexOf   abc     tommy can eat   8       Returns 11 reLastIndexOf   abc     tommy can eat       Returns 11   You could also prototype the functions onto the RegExp object   RegExp prototype indexOf   function str  startIndex        var re   new RegExp this source   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      RegExp prototype lastIndexOf   function str  startIndex        var src         test this source   amp  amp           test this source    this source   this source          S  s      this source            var re   new RegExp src   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length         abc   indexOf  tommy can eat        Returns 6   abc   indexOf  tommy can eat   8       Returns 11   abc   lastIndexOf  tommy can eat       Returns 11   A quick explanation of how I am modifying the RegExp   For indexOf I just have to ensure that the global flag is set   For lastIndexOf of I am using a negative look-ahead to find the last occurrence unless the RegExp was already matching at the end of the string

User · Answer

If you are looking for a very simple lastIndex lookup with RegExp and don t care if it mimics lastIndexOf to the last detail  this may catch your attention   I simply reverse the string  and subtract the first occurence index from length - 1  It happens to pass my test  but I think there could arise a performance issue with long strings   interface String     reverse    string    lastIndex regex  RegExp   number     String prototype reverse   function this  string      return this split          reverse        join          String prototype lastIndex   function this  string  regex  RegExp      const exec   regex exec this reverse       return exec     null   -1   this length - 1 - exec index

User · Answer

In certain simple cases  you can simplify your backwards search by using split   function regexlast string re     var tokens string split re     return  tokens length gt 1   string length-tokens tokens length-1  length  null      This has a few serious problems    overlapping matches won t show up returned index is for the end of the match rather than the beginning  fine if your regex is a constant    But on the bright side it s way less code  For a constant-length regex that can t overlap  like   s w  for finding word boundaries  this is good enough

User · Answer

RexExp instances have a lastIndex property already  if they are global  and so what I m doing is copying the regular expression  modifying it slightly to suit our purposes  exec-ing it on the string and looking at the lastIndex   This will inevitably be faster than looping on the string    You have enough examples of how to put this onto the string prototype  right    function reIndexOf reIn  str  startIndex        var re   new RegExp reIn source   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      function reLastIndexOf reIn  str  startIndex        var src         test reIn source   amp  amp           test reIn source    reIn source   reIn source          S  s      reIn source            var re   new RegExp src   g     reIn ignoreCase    i           reIn multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      reIndexOf   abc     tommy can eat        Returns 6 reIndexOf   abc     tommy can eat   8       Returns 11 reLastIndexOf   abc     tommy can eat       Returns 11   You could also prototype the functions onto the RegExp object   RegExp prototype indexOf   function str  startIndex        var re   new RegExp this source   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length      RegExp prototype lastIndexOf   function str  startIndex        var src         test this source   amp  amp           test this source    this source   this source          S  s      this source            var re   new RegExp src   g     this ignoreCase    i           this multiLine    m              re lastIndex   startIndex    0      var res   re exec str       if  res  return -1      return re lastIndex - res 0  length         abc   indexOf  tommy can eat        Returns 6   abc   indexOf  tommy can eat   8       Returns 11   abc   lastIndexOf  tommy can eat       Returns 11   A quick explanation of how I am modifying the RegExp   For indexOf I just have to ensure that the global flag is set   For lastIndexOf of I am using a negative look-ahead to find the last occurrence unless the RegExp was already matching at the end of the string

User · Answer

Combining a few of the approaches already mentioned  the indexOf is obviously rather simple   I think these are the functions that will do the trick  function regexIndexOf string  regex  startpos        var indexOf   string substring startpos    0  search regex       return  indexOf  gt   0     indexOf    startpos    0     indexOf     function regexLastIndexOf string  regex  startpos        regex    regex global    regex   new RegExp regex source   quot g quot     regex ignoreCase    quot i quot     quot  quot      regex multiLine    quot m quot     quot  quot         if typeof  startpos      quot undefined quot             startpos   string length        else if startpos  lt  0            startpos   0            var stringToWorkWith   string substring 0  startpos   1       var lastIndexOf   -1      var nextStop   0      while  result   regex exec stringToWorkWith      null            lastIndexOf   result index          regex lastIndex     nextStop            return lastIndexOf      UPDATE  Edited regexLastIndexOf   so that is seems to mimic lastIndexOf   now  Please let me know if it still fails and under what circumstances   UPDATE  Passes all tests found on in comments on this page  and my own  Of course  that doesn t mean it s bulletproof  Any feedback appreciated

User · Answer

Based on BaileyP s answer  The main difference is that these methods return -1 if the pattern can t be matched   Edit  Thanks to Jason Bunting s answer I got an idea  Why not modify the  lastIndex property of the regex  Though this will only work for patterns with the global flag   g    Edit  Updated to pass the test-cases   String prototype regexIndexOf   function re  startPos        startPos   startPos    0       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              re lastIndex   startPos      var match   re exec this        if  match  return match index      else return -1     String prototype regexLastIndexOf   function re  startPos        startPos   startPos     undefined   this length   startPos       if   re global            var flags    g     re multiline  m         re ignoreCase  i               re   new RegExp re source  flags              var lastSuccess   -1      for  var pos   0  pos  lt   startPos  pos              re lastIndex   pos           var match   re exec this           if   match  break           pos   match index          if  pos  lt   startPos  lastSuccess   pos             return lastSuccess

User · Answer

Well  as you are just looking to match the position of a character   regex is possibly overkill    I presume all you want is  instead of  find first of these this character    just find first of these characters    This of course is the simple answer  but does what your question sets out to do  albeit without the regex part   because you didn t clarify why specifically it had to be a regex    function mIndexOf  str   chars  offset        var first    -1      for  var i   0  i  lt  chars length   i                var p   str indexOf  chars i    offset           if  p  lt  first    first     -1                      first   p                  return first     String prototype mIndexOf   function  chars  offset        return mIndexOf  this  chars  offset      I m really averse to monkey patching       mIndexOf   hello world     a   o   w    0     gt  gt  4  mIndexOf   hello world     a    0     gt  gt  -1  mIndexOf   hello world     a   o   w    4     gt  gt  4 mIndexOf   hello world     a   o   w    5     gt  gt  6 mIndexOf   hello world     a   o   w    7     gt  gt  -1  mIndexOf   hello world     a   o   w   d    7     gt  gt  10 mIndexOf   hello world     a   o   w   d    10     gt  gt  10 mIndexOf   hello world     a   o   w   d    11     gt  gt  -1

User · Answer

I needed a regexIndexOf function also for an array  so I programed one myself  However I doubt  that it s optimized  but i guess it should work properly   Array prototype regexIndexOf   function  regex  startpos   0        len   this length      for x   startpos  x  lt  len  x             if typeof this x      undefined   amp  amp      this x   match regex                return x                      return -1     arr       arr push null   arr push NaN   arr 3    7  arr push  asdf    arr push  qwer    arr push 9   arr push         console log arr   arr regexIndexOf   d   4

User · Answer

Use   str search regex    See the documentation here

User · Answer

You could use substr   str substr i  match   abc

User · Answer

Combining a few of the approaches already mentioned  the indexOf is obviously rather simple   I think these are the functions that will do the trick  function regexIndexOf string  regex  startpos        var indexOf   string substring startpos    0  search regex       return  indexOf  gt   0     indexOf    startpos    0     indexOf     function regexLastIndexOf string  regex  startpos        regex    regex global    regex   new RegExp regex source   quot g quot     regex ignoreCase    quot i quot     quot  quot      regex multiLine    quot m quot     quot  quot         if typeof  startpos      quot undefined quot             startpos   string length        else if startpos  lt  0            startpos   0            var stringToWorkWith   string substring 0  startpos   1       var lastIndexOf   -1      var nextStop   0      while  result   regex exec stringToWorkWith      null            lastIndexOf   result index          regex lastIndex     nextStop            return lastIndexOf      UPDATE  Edited regexLastIndexOf   so that is seems to mimic lastIndexOf   now  Please let me know if it still fails and under what circumstances   UPDATE  Passes all tests found on in comments on this page  and my own  Of course  that doesn t mean it s bulletproof  Any feedback appreciated

User · Answer

After having all the proposed solutions fail my tests one way or the other   edit  some were updated to pass the tests after I wrote this  I found the mozilla implementation for Array indexOf and Array lastIndexOf  I used those to implement my version of String prototype regexIndexOf and String prototype regexLastIndexOf as follows   String prototype regexIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1      0      from    from  lt  0    Math ceil from    Math floor from       if  from  lt  0        from    len       for    from  lt  len  from            if  from in arr  amp  amp  elt exec arr from              return from            return -1      String prototype regexLastIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1        if  isNaN from           from   len - 1        else         from    from  lt  0    Math ceil from    Math floor from         if  from  lt  0          from    len        else if  from  gt   len          from   len - 1             for    from  gt  -1  from--          if  from in arr  amp  amp  elt exec arr from             return from            return -1         They seem to pass the test functions I provided in the question   Obviously they only work if the regular expression matches one character but that is enough for my purpose since I will be using it for things like    abc      s    W    D    I will keep monitoring the question in case someone provides a better faster cleaner more generic implementation that works on any regular expression

User · Answer

Combining a few of the approaches already mentioned  the indexOf is obviously rather simple   I think these are the functions that will do the trick  function regexIndexOf string  regex  startpos        var indexOf   string substring startpos    0  search regex       return  indexOf  gt   0     indexOf    startpos    0     indexOf     function regexLastIndexOf string  regex  startpos        regex    regex global    regex   new RegExp regex source   quot g quot     regex ignoreCase    quot i quot     quot  quot      regex multiLine    quot m quot     quot  quot         if typeof  startpos      quot undefined quot             startpos   string length        else if startpos  lt  0            startpos   0            var stringToWorkWith   string substring 0  startpos   1       var lastIndexOf   -1      var nextStop   0      while  result   regex exec stringToWorkWith      null            lastIndexOf   result index          regex lastIndex     nextStop            return lastIndexOf      UPDATE  Edited regexLastIndexOf   so that is seems to mimic lastIndexOf   now  Please let me know if it still fails and under what circumstances   UPDATE  Passes all tests found on in comments on this page  and my own  Of course  that doesn t mean it s bulletproof  Any feedback appreciated

User · Answer

For data with sparse matches  using string search is the fastest across browsers  It re-slices a string each iteration to    function lastIndexOfSearch string  regex  index      if index     0    index       string   string slice 0  Math max 0 index      var idx    var offset   -1    while   idx   string search regex       -1        offset    idx   1      string   string slice idx   1         return offset      For dense data I made this  It s complex compared to the execute method  but for dense data  it s 2-10x faster than every other method I tried  and about 100x faster than the accepted solution  The main points are    It calls exec on the regex passed in once to verify there is a match or quit early  I do this using     in a similar method  but on IE checking with exec is dramatically faster  It constructs and caches a modified regex in the format   r       r   The new regex is executed and the results from either that exec  or the first exec  are returned   function lastIndexOfGroupSimple string  regex  index        if  index     0    index  string   string slice 0  Math max 0  index   1        regex lastIndex   0      var lastRegex  index     flags    g     regex multiline    m           regex ignoreCase    i             key   regex source         flags      match   regex exec string       if   match  return -1      if  lastIndexOfGroupSimple cache     undefined  lastIndexOfGroupSimple cache           lastRegex   lastIndexOfGroupSimple cache key       if   lastRegex          lastIndexOfGroupSimple cache key    lastRegex   new RegExp         regex source               regex source        flags       index   match index      lastRegex lastIndex   match index      return  match   lastRegex exec string     lastRegex lastIndex - match 1  length   index        jsPerf of methods  I don t understand the purpose of the tests up top  Situations that require a regex are impossible to compare against a call to indexOf  which I think is the point of making the method in the first place  To get the test to pass  it makes more sense to use  xxx    x    than adjust the way the regex iterates

User · Answer

Combining a few of the approaches already mentioned  the indexOf is obviously rather simple   I think these are the functions that will do the trick  function regexIndexOf string  regex  startpos        var indexOf   string substring startpos    0  search regex       return  indexOf  gt   0     indexOf    startpos    0     indexOf     function regexLastIndexOf string  regex  startpos        regex    regex global    regex   new RegExp regex source   quot g quot     regex ignoreCase    quot i quot     quot  quot      regex multiLine    quot m quot     quot  quot         if typeof  startpos      quot undefined quot             startpos   string length        else if startpos  lt  0            startpos   0            var stringToWorkWith   string substring 0  startpos   1       var lastIndexOf   -1      var nextStop   0      while  result   regex exec stringToWorkWith      null            lastIndexOf   result index          regex lastIndex     nextStop            return lastIndexOf      UPDATE  Edited regexLastIndexOf   so that is seems to mimic lastIndexOf   now  Please let me know if it still fails and under what circumstances   UPDATE  Passes all tests found on in comments on this page  and my own  Of course  that doesn t mean it s bulletproof  Any feedback appreciated

User · Answer

You could use substr   str substr i  match   abc

User · Answer

After having all the proposed solutions fail my tests one way or the other   edit  some were updated to pass the tests after I wrote this  I found the mozilla implementation for Array indexOf and Array lastIndexOf  I used those to implement my version of String prototype regexIndexOf and String prototype regexLastIndexOf as follows   String prototype regexIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1      0      from    from  lt  0    Math ceil from    Math floor from       if  from  lt  0        from    len       for    from  lt  len  from            if  from in arr  amp  amp  elt exec arr from              return from            return -1      String prototype regexLastIndexOf   function elt     from            var arr   this split          var len   arr length       var from   Number arguments 1        if  isNaN from           from   len - 1        else         from    from  lt  0    Math ceil from    Math floor from         if  from  lt  0          from    len        else if  from  gt   len          from   len - 1             for    from  gt  -1  from--          if  from in arr  amp  amp  elt exec arr from             return from            return -1         They seem to pass the test functions I provided in the question   Obviously they only work if the regular expression matches one character but that is enough for my purpose since I will be using it for things like    abc      s    W    D    I will keep monitoring the question in case someone provides a better faster cleaner more generic implementation that works on any regular expression

User · Answer

In certain simple cases  you can simplify your backwards search by using split   function regexlast string re     var tokens string split re     return  tokens length gt 1   string length-tokens tokens length-1  length  null      This has a few serious problems    overlapping matches won t show up returned index is for the end of the match rather than the beginning  fine if your regex is a constant    But on the bright side it s way less code  For a constant-length regex that can t overlap  like   s w  for finding word boundaries  this is good enough

User · Answer

Instances of the String constructor have a  search   method which accepts a RegExp and returns the index of the first match   To start the search from a particular position  faking the second parameter of  indexOf    you can slice off the first i characters   str slice i  search  re     But this will get the index in the shorter string  after the first part was sliced off  so you ll want to then add the length of the chopped off part  i  to the returned index if it wasn t -1  This will give you the index in the original string   function regexIndexOf text  re  i        var indexInSuffix   text slice i  search re       return indexInSuffix  lt  0   indexInSuffix   indexInSuffix   i

[javascript] Is there a version of JavaScript's String.indexOf() that allows for regular expressions?

Examples related to javascript

Examples related to regex

Examples related to indexof