Javascript - Add to a RegEx

Question

I'm working with a library which specifies a RegEx on an object. I need to add some characters into that RegEx. Is there a less hacky way of doing this?

I have no control over this.stripCharsRegex.

// this.stripCharsRegex =   /[^0123456789.-]/gi
var regexChars = this.stripCharsRegex.toString();
regexChars = regexChars.substr(3,regex.length-7);  // remove '/[^'  and   ']/gi'
var regex = new RegExp('[^£$€'+regexChars+']','gi');  // (e.g.)
this.stripCharsRegex = regex;

Not really less hacky but you can tidy by chaining lines 1 with 2 and 3 with 4. — dewd
– dewd, Commented Nov 25, 2014 at 20:36
You can get the regex with .source(), but this is a read-only property. You'd have to make a new regex object. — Mike
– Mike, Commented Nov 25, 2014 at 20:37

dfsq · Accepted Answer · 2014-11-25 21:23:23Z

4

I think you should be able to combine new regexp rule with old RegExp object if you use its source property. Try this:

this.stripCharsRegex = new RegExp('(?=[^£$€])' + this.stripCharsRegex.source, 'gi');

Check test snippet below.

var stripCharsRegex = new RegExp('[^0123456789.-]', 'gi');

alert( 'Total: $123 test - £80&dd'.replace(stripCharsRegex, '') );

// extend regexp by creating new RegExp object
stripCharsRegex = new RegExp('(?=[^£$€])' + stripCharsRegex.source, 'gi');

alert( 'Total: $123 test - £80&dd'.replace(stripCharsRegex, '') );

edited Nov 25, 2014 at 21:23

answered Nov 25, 2014 at 21:18

dfsq

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3 Comments

Dave Salomon Over a year ago

Great - .source will be what I'm looking for. Nice work with the positive-lookahead too - thanks!

Ruan Mendes Over a year ago

You can also figure out if the regex is global and case sensitive, instead of duplicating the gi flags

dfsq Over a year ago

@JuanMendes Yes, it's true, we can check global and ignoreCase properties.

Ruan Mendes · Accepted Answer · 2014-11-25 21:48:45Z

Here's a generic way to add a character class to any regular expression

function addCharacterClass(regex, string) {
    var flags = (regex.global ? "g" : "" ) + (regex.ignoreCase ? "i" : "" )
    return new RegExp(regex.source + "|([" + string + "])", flags);
}

document.getElementById('check').addEventListener('click', function(){
   var originalRegex = new RegExp(document.getElementById('regex').value);
   var additionalChars = document.getElementById('chars').value;
   var testString = document.getElementById('test-string').value;
   alert("Does regex match? " + addCharacterClass(originalRegex, additionalChars).test(testString) );
  
});

Original Regex: <input id="regex" value="[^0123456789.-]" /> <br />
Additional Characters: <input id="chars" value="£$€"/> <br />
Test string <input id="test-string" value="$123456"> <br />
<button id="check">Check</button>

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