Converting user input string to regular expression

Question

I am designing a regular expression tester in HTML and JavaScript  The user will enter a regex  a string  and choose the function they want to test with  e g  search  match  replace  etc   via radio button and the program will display the results when that function is run with the specified arguments  Naturally there will be extra text boxes for the extra arguments to replace and such    My problem is getting the string from the user and turning it into a regular expression  If I say that they don t need to have    s around the regex they enter  then they can t set flags  like g and i  So they have to have the    s around the expression  but how can I convert that string to a regex  It can t be a literal since its a string  and I can t pass it to the RegExp constructor since its not a string without the    s  Is there any other way to make a user input string into a regex  Will I have to parse the string and flags of the regex with the    s then construct it another way  Should I have them enter a string  and then enter the flags separately

User · Answer

I use eval to solve this problem   For example       function regex exec                 Important  Like  Samuel Faure mentioned  Eval on user input is a crazy security risk  so before use this method  please take care of the security risk           var regex       regex   val                eval           var patt   eval userInput                result   val patt exec     textContent   val

User · Answer

var flags   inputstring replace        gimy         1    var pattern   inputstring replace new RegExp            flags         1    var regex   new RegExp pattern  flags     or  var match   inputstring match new RegExp            gimy            sanity check here var regex   new RegExp match 1   match 2

User · Answer

I suggest you also add separate checkboxes or a textfield for the special flags  That way it is clear that the user does not need to add any    s  In the case of a replace  provide two textfields  This will make your life a lot easier   Why  Because otherwise some users will add    s while other will not  And some will make a syntax error  Then  after you stripped the    s  you may end up with a syntactically valid regex that is nothing like what the user intended  leading to strange behaviour  from the user s perspective

User · Answer

Try using the following function  const stringToRegex   str   gt           Main regex     const main   str match               1              Regex options     const options   str match               1              Compiled regex     return new RegExp main  options     You can use it like so   quot abc quot  match stringToRegex  quot  a g quot        gt    quot a quot

User · Answer

Here is a one-liner  str replace                     g       amp     I got it from the escape-string-regexp NPM module   Trying it out   escapeStringRegExp matchOperatorsRe                       g  function escapeStringRegExp str        return str replace escapeStringRegExp matchOperatorsRe       amp        console log new RegExp escapeStringRegExp  example com           gt   example  com    Using tagged template literals with flags support   function str2reg flags    u         return     args    gt  new RegExp escapeStringRegExp evalTemplate    args             flags     function evalTemplate strings     values        let i   0     return strings reduce  str  string    gt     str   string            i  lt  values length   values i                   console log str2reg   example com        gt   example  com u

User · Answer

Thanks to earlier answers  this blocks serves well as a general purpose solution for applying a configurable string into a RegEx    for filtering text   var permittedChars     a-z0-9         lt  gt    permittedChars         permittedChars         var flags    gi   var strFilterRegEx   new RegExp permittedChars  flags    log debug   strFilterRegEx      strFilterRegEx    strVal   strVal replace strFilterRegEx          this replaces hard code solt     strVal   strVal replace    a-z0-9          ig

User · Answer

You can ask for flags using checkboxes then do something like this   var userInput   formInput  var flags       if formGlobalCheckboxChecked  flags     g   if formCaseICheckboxChecked  flags     i   var reg   new RegExp userInput  flags

User · Answer

In my case the user input somethimes was sorrounded by delimiters and sometimes not  therefore I added another case    var regParts   inputstring match              gim        if  regParts           the parsed pattern had delimiters and modifiers  handle them       var regexp   new RegExp regParts 1   regParts 2      else          we got pattern string without delimiters     var regexp   new RegExp inputstring

User · Answer

Use the RegExp object constructor to create a regular expression from a string   var re   new RegExp  a b    i       same as var re    a b i

User · Answer

Use the JavaScript RegExp object constructor   var re   new RegExp    w     re test  hello      You can pass flags as a second string argument to the constructor  See the documentation for details

User · Answer

This will work also when the string is invalid or does not contain flags etc    x000D   x000D  function regExpFromString q    x000D    let flags   q replace        gimuy         1    x000D    if  flags     q  flags       x000D    let pattern    flags   q replace new RegExp              flags           1     q   x000D    try   return new RegExp pattern  flags     catch  e    return null    x000D    x000D   x000D  console log regExpFromString    bword  b     x000D  console log regExpFromString      bword  b  gi     x000D               x000D   x000D   x000D

[javascript] Converting user input string to regular expression

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