Dynamic Regular Expressions in Javascript

I'd just to give credit to a great post by John-Paul Bader about Dynamic Regular Expressions in Javascript. It saved a few hours of searching around for obsolete documentation.

Using regular expressions is easy enough...

var result String('The yellow fox has big eyes').match(/\b\w+\b/gi);

But it isn't so straight-forward when you want to create a regular expression on the fly.
Let's say I want to find all words beginning with a certain letter.

var letter = 'y';

You cannot do any of the following (warning, bad code ahead!)

var result String('The yellow fox has big eyes').match(/\bletter\w+\b/gi);
var result String('The yellow fox has big eyes').match(/\b/ + letter + /\w+\b/gi);
var result String('The yellow fox has big eyes').match(/\b' + letter + '\w+\b/gi);

The only way to make it work is to compile a new regular expression during run-time, using the RegExp object. I won't into details, but the syntax is simple:

var rx = new RegExp(String pattern, String options);

Where pattern is you expression and options are your usual g (global), i (ignore case), m (multi-line) switches.

So you'd expect this to work...

var search = new RegExp('\b'+letter+'\w+\b', 'gi');

But that still isn't quite right. The backslash is a special escape character in Javascript strings. This causes the code above to build a regular expression like this byw+b, but what we actually want is this: \by\w+\b.

Which brings us to the final step in this journey. The correct way to work with truly dynamic regular expressions is...

var search = new RegExp('\\b'+letter+'\\w+\\b', 'gi');

So, to sum up:

1. Put your regular expression in a string,
   ie: var pattern = '\b\w+\b';
   
   
2. Escape Javascript's escape characters,
   ie: var pattern = '\\b\\w+\\b';
   
   
3. Use the RegExp builder to compile a regular expression on the fly,
   ie: test = new RegExp(pattern, 'gi');
   

And that's it!