How do you use a variable in a regular expression?


I would like to create a String.replaceAll() method in JavaScript and I'm thinking that using a regex would be most terse way to do it. However, I can't figure out how to pass a variable in to a regex. I can do this already which will replace all the instances of "B" with "A".

"ABABAB".replace(/B/g, "A");

But I want to do something like this:

String.prototype.replaceAll = function(replaceThis, withThis) {
    this.replace(/replaceThis/g, withThis);
};

But obviously this will only replace the text "replaceThis"...so how do I pass this variable in to my regex string?

Instead of using the /regex/g syntax, you can construct a new RegExp object:

var replace = "regex";
var re = new RegExp(replace,"g");

You can dynamically create regex objects this way. Then you will do:

"mystring".replace(re, "newstring");

As Eric Wendelin mentioned, you can do something like this:

str1 = "pattern"
var re = new RegExp(str1, "g");
"pattern matching .".replace(re, "regex");

This yields "regex matching .". However, it will fail if str1 is ".". You'd expect the result to be "pattern matching regex", replacing the period with "regex", but it'll turn out to be...

regexregexregexregexregexregexregexregexregexregexregexregexregexregexregexregexregexregex

This is because, although "." is a String, in the RegExp constructor it's still interpreted as a regular expression, meaning any non-line-break character, meaning every character in the string. For this purpose, the following function may be useful:

 RegExp.quote = function(str) {
     return str.replace(/([.?*+^$[\]\\(){}|-])/g, "\\$1");
 };

Then you can do:

str1 = "."
var re = new RegExp(RegExp.quote(str1), "g");
"pattern matching .".replace(re, "regex");

yielding "pattern matching regex".


"ABABAB".replace(/B/g, "A");

As always: don't use regex unless you have to. For a simple string replace, the idiom is:

'ABABAB'.split('B').join('A')

Then you don't have to worry about the quoting issues mentioned in Gracenotes's answer.


For anyone looking to use variable with the match method, this worked for me

var alpha = 'fig';
'food fight'.match(alpha + 'ht')[0]; // fight

This:

var txt=new RegExp(pattern,attributes);

is equivalent to this:

var txt=/pattern/attributes;

See http://www.w3schools.com/jsref/jsref_obj_regexp.asp.


If you want to get ALL occurrences (g), be case insensitive (i), and use boundaries so that it isn't a word within another word (\\b):

re = new RegExp(`\\b${replaceThis}\\b`, 'gi');

Example:

let inputString = "I'm John, or johnny, but I prefer john.";
let replaceThis = "John";
let re = new RegExp(`\\b${replaceThis}\\b`, 'gi');
console.log(inputString.replace(re, "Jack")); // I'm Jack, or johnny, but I prefer Jack.

this.replace( new RegExp( replaceThis, 'g' ), withThis );

You want to build the regular expression dynamically and for this the proper solutuion is to use the new RegExp(string) constructor. In order for constructor to treat special characters literally, you must escape them. There is a built-in function in jQuery UI autocomplete widget called $.ui.autocomplete.escapeRegex:

[...] you can make use of the built-in $.ui.autocomplete.escapeRegex function. It'll take a single string argument and escape all regex characters, making the result safe to pass to new RegExp().

If you are using jQuery UI you can use that function, or copy its definition from the source:

function escapeRegex( value ) {
    return value.replace( /[\-\[\]{}()*+?.,\\\^$|#\s]/g, "\\$&" );
}

And use it like this:

"[z-a][z-a][z-a]".replace(new RegExp(escapeRegex("[z-a]"), "g"), "[a-z]");
//            escapeRegex("[z-a]")       -> "\[z\-a\]"
// new RegExp(escapeRegex("[z-a]"), "g") -> /\[z\-a\]/g
// end result                            -> "[a-z][a-z][a-z]"

String.prototype.replaceAll = function (replaceThis, withThis) {
   var re = new RegExp(replaceThis,"g"); 
   return this.replace(re, withThis);
};
var aa = "abab54..aba".replaceAll("\\.", "v");

Test with this tool


String.prototype.replaceAll = function(a, b) {
    return this.replace(new RegExp(a.replace(/([.?*+^$[\]\\(){}|-])/ig, "\\$1"), 'ig'), b)
}

Test it like:

var whatever = 'Some [b]random[/b] text in a [b]sentence.[/b]'

console.log(whatever.replaceAll("[", "<").replaceAll("]", ">"))

Here's another replaceAll implementation:

    String.prototype.replaceAll = function (stringToFind, stringToReplace) {
        if ( stringToFind == stringToReplace) return this;
        var temp = this;
        var index = temp.indexOf(stringToFind);
        while (index != -1) {
            temp = temp.replace(stringToFind, stringToReplace);
            index = temp.indexOf(stringToFind);
        }
        return temp;
    };

And the coffeescript version of Steven Penny's answer, since this is #2 google result....even if coffee is just javascript with a lot of characters removed...;)

baz = "foo"
filter = new RegExp(baz + "d")
"food fight".match(filter)[0] // food

and in my particular case

robot.name=hubot
filter = new RegExp(robot.name)
if msg.match.input.match(filter)
  console.log "True!"

While you can make dynamically-created RegExp's (as per the other responses to this question), I'll echo my comment from a similar post: The functional form of String.replace() is extremely useful and in many cases reduces the need for dynamically-created RegExp objects. (which are kind of a pain 'cause you have to express the input to the RegExp constructor as a string rather than use the slashes /[A-Z]+/ regexp literal format)


To satisfy my need to insert a variable/alias/function into a Regular Expression, this is what I came up with:

oldre = /xx\(""\)/;
function newre(e){
    return RegExp(e.toString().replace(/\//g,"").replace(/xx/g, yy), "g")
};

String.prototype.replaceAll = this.replace(newre(oldre), "withThis");

where 'oldre' is the original regexp that I want to insert a variable, 'xx' is the placeholder for that variable/alias/function, and 'yy' is the actual variable name, alias, or function.


You can use this if $1 not work with you

var pattern = new RegExp("amman","i");
"abc Amman efg".replace(pattern,"<b>"+"abc Amman efg".match(pattern)[0]+"</b>");

You can always use indexOf repeatedly:

String.prototype.replaceAll = function(substring, replacement) {
    var result = '';
    var lastIndex = 0;

    while(true) {
        var index = this.indexOf(substring, lastIndex);
        if(index === -1) break;
        result += this.substring(lastIndex, index) + replacement;
        lastIndex = index + substring.length;
    }

    return result + this.substring(lastIndex);
};

This doesn’t go into an infinite loop when the replacement contains the match.


Your solution is here:

Pass a variable to regular expression.

The one which I have implemented is by taking the value from a text field which is the one you want to replace and another is the "replace with" text field, getting the value from text-field in a variable and setting the variable to RegExp function to further replace. In my case I am using Jquery, you also can do it by only JavaScript too.

JavaScript code:

  var replace =document.getElementById("replace}"); // getting a value from a text field with I want to replace
  var replace_with = document.getElementById("with"); //Getting the value from another text fields with which I want to replace another string.

  var sRegExInput = new RegExp(replace, "g");    
  $("body").children().each(function() {
    $(this).html($(this).html().replace(sRegExInput,replace_with));
  });

This code is on Onclick event of a button, you can put this in a function to call.

So now You can pass variable in replace function.


None of these answers were clear to me. I eventually found a good explanation at http://burnignorance.com/php-programming-tips/how-to-use-a-variable-in-replace-function-of-javascript/

The simple answer is:

var search_term = new RegExp(search_term, "g");    
text = text.replace(search_term, replace_term);

For example:

$("button").click(function() {
  Find_and_replace("Lorem", "Chocolate");
  Find_and_replace("ipsum", "ice-cream");
});

function Find_and_replace(search_term, replace_term) {
  text = $("textbox").html();
  var search_term = new RegExp(search_term, "g");
  text = text.replace(search_term, replace_term);
  $("textbox").html(text);
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<textbox>
  Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum Lorem ipsum
</textbox>
<button>Click me</button>


For multiple replace without regular expressions I went with the following:

      let str = "I am a cat man. I like cats";
      let find = "cat";
      let replace = "dog";


      // Count how many occurrences there are of the string to find 
      // inside the str to be examined.
      let findCount = str.split(find).length - 1;

      let loopCount = 0;

      while (loopCount < findCount) 
      {
        str = str.replace(find, replace);
        loopCount = loopCount + 1;
      }  

      console.log(str);
      // I am a dog man. I like dogs

The important part of the solution was found here


This self calling function will iterate over replacerItems using an index, and change replacerItems[index] globally on the string with each pass.

  const replacerItems = ["a", "b", "c"];    

    function replacer(str, index){
          const item = replacerItems[index];
          const regex = new RegExp(`[${item}]`, "g");
          const newStr = str.replace(regex, "z");
          if (index < replacerItems.length - 1) {
            return replacer(newStr, index + 1);
          }
          return newStr;
    }

// console.log(replacer('abcdefg', 0)) will output 'zzzdefg'