Use dynamic variable string as regex pattern in JavaScript

Question

I want to add a  variable  tag to values with regex  the pattern works fine with PHP but I have troubles implementing it into JavaScript   The pattern is  value is the variable              lt    gt     gt    lt   a gt    b value  b is   I escaped the backslashes   var str       div   html    var regex              lt    gt     gt    lt    a gt     b     value       b is       div   html str replace regex    lt a href       value    gt     value     lt  a gt        But this seem not to be right  I logged the pattern and its exactly what it should be  Any ideas

User · Answer

I found I had to double slash the  b to get it working  For example to remove  1x  words from a string using a variable  I needed to use       str    1x       var regex   new RegExp    b  str    b   g       same as inv replace   b1x b g          inv inv replace regex

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I found this thread useful - so I thought I would add the answer to my own problem   I wanted to edit a database configuration file  datastax cassandra  from a node application in javascript and for one of the settings in the file I needed to match on a string and then replace the line following it    This was my solution    dse cassandra yaml   etc dse cassandra cassandra yaml      a  find the searchString and grab all text on the following line to it    b  replace all next line text with a newString supplied to function    note - leaves searchString text untouched function replaceStringNextLine file  searchString  newString    fs readFile file   utf-8   function err  data   if  err  throw err         need to use double escape      when putting regex in strings       var re      s    -  s           s          var myRegExp   new RegExp searchString   re   g        var match   myRegExp exec data       var replaceThis   match 1       var writeString   data replace replaceThis  newString       fs writeFile file  writeString   utf-8   function  err        if  err  throw err          console log file     updated                   searchString    data file directories   newString    -  mnt cassandra data   replaceStringNextLine dse cassandra yaml  searchString  newString      After running  it will change the existing data directory setting to the new one   config file before   data file directories       -  var lib cassandra data   config file after   data file directories    -  mnt cassandra data

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Using string variable s  content as part of a more complex composed regex expression  es6 ts  This example will replace all urls using my-domain com to my-other-domain  both are variables   You can do dynamic regexs by combining string values and other regex expressions within a raw string template  Using String raw will prevent javascript from escaping any character within your string values     Strings with some data const domainStr    my-domain com  const newDomain    my-other-domain com      Make sure your string is regex friendly    This will replace dots for      const regexUrl       gm      const substr           const domain   domainStr replace regexUrl  substr      domain is a regex friendly string   my-domain  com  console log  Regex expresion for domain   domain      HERE    You can  assemble a complex regex using string pieces  const re   new RegExp  String raw         quot  https         domain    S       quot       gm        now I ll use the regex expression groups to replace the domain const domainSubst     1  newDomain  3       const page contains all the html text const result   page replace re  domainSubst     note  Don t forget to use regex101 com to create  test and export REGEX code

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You don t need the   to define a regular expression so just   var regex             lt    gt     gt    lt   a gt    b value  b is     this is valid syntax   If value is a variable and you want a dynamic regular expression then you can t use this notation  use the alternative notation   String replace also accepts strings as input  so you can do  fox  replace  fox    bear     Alternative   var regex   new RegExp            lt    gt     gt    lt   a gt    b value  b     is    var regex   new RegExp            lt    gt     gt    lt   a gt    b     value      b     is    var regex   new RegExp            lt    gt     gt    lt   a gt    b      b     is      Keep in mind that if value contains regular expressions characters like      and   you will need to escape them

User · Answer

If you are trying to use a variable value in the expression  you must use the RegExp  constructor    var regex           lt    gt     gt    lt   a gt    b     value      b   new RegExp regex   is

User · Answer

var string    Hi welcome to stack overflow  var toSearch    stack     case insensitive search  var result   string search new RegExp toSearch   i     gt  0    Matched     notMatched    https   jsfiddle net 9f0mb6Lz   Hope this helps

User · Answer

Much easier way  use template literals  var variable    foo  var expression        variable     var re   new RegExp expression   g   re test  fdjklsffoodjkslfd      true re test  fdjklsfdjkslfd      false

User · Answer

To create the regex from a string  you have to use JavaScript s RegExp object  If you also want to match replace more than one time  then you must add the g  global match  flag  Here s an example  var stringToGoIntoTheRegex    quot abc quot   var regex   new RegExp  quot   quot    stringToGoIntoTheRegex    quot   quot    quot g quot       at this point  the line above is the same as  var regex     abc  g   var input    quot Hello this is  abc  some  abc  stuff  quot   var output   input replace regex   quot    quot    alert output      Hello this is    some    stuff   JSFiddle demo here   In the general case  escape the string before using as regex  Not every string is a valid regex  though  there are some speciall characters  like   or    To work around this issue  simply escape the string before turning it into a regex  A utility function for that goes in the sample below  function escapeRegExp stringToGoIntoTheRegex        return stringToGoIntoTheRegex replace   -                    g       amp        var stringToGoIntoTheRegex   escapeRegExp  quot abc quot       this is the only change from above var regex   new RegExp  quot   quot    stringToGoIntoTheRegex    quot   quot    quot g quot       at this point  the line above is the same as  var regex     abc  g   var input    quot Hello this is  abc  some  abc  stuff  quot   var output   input replace regex   quot    quot    alert output      Hello this is    some    stuff   JSFiddle demo here    Note  the regex in the question uses the s modifier  which didn t exist at the time of the question  but does exist -- a s  dotall  flag modifier in JavaScript -- today

[javascript] Use dynamic (variable) string as regex pattern in JavaScript

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