How to use variables in regular expressions?

Question

I want to create a String.replaceAll() method in JavaScript and I think using regular expressions is the cleanest way. However, I don't know how to pass variables to the regular expression. I can already do this, replacing all 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 apparently this will only replace the text "replaceThis"... So how do I pass this variable into my regex string?

Answer 1

As Eric Wendelin mentioned, you can do this:

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

This produces "Regular expression match." . However, it fails if str1 is ".". You expected the result to be "Pattern Matching Regular Expression", replacing the period with "Regular Expression", but the result would be...

regexregexregexregexregexregexregexregexregexregexregexregexregexregexregexregexregexregex

This is because, although "." is a string, in the RegExp constructor it is still interpreted as a regular expression, representing any non-newline character, representing each character in the string character. For this purpose, the following functions may be useful:

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

Then you can do this:

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

Produces "pattern matching regular expression" .

Answer 2

You can construct a new RegExp object:

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

You can dynamically create regular expression objects this way. Then you would do:

"mystring1".replace(re, "newstring");