How do you use a variable in a regular expression? – Dev

The best answers to the question “How do you use a variable in a regular expression?” in the category Dev.

QUESTION:

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?

ANSWER:

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".

ANSWER:

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

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

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

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

ANSWER:

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.

ANSWER:

"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.