Question

Can you please help me. How can I add this regex (?<=^|\s):d(?=$|\s) in javascript RegExp?

e.g

regex = new RegExp("?????" , 'g');

I want to replace the emoticon :d, but only if it is surrounded by spaces (or at an end of the string).

Answer 1

Firstly, as Some1.Kill.The.DJ mentioned, I recommend you use the literal syntax to create the regular expression:

var pattern = /yourPatternHere/g;

It’s shorter, easier to read and you avoid complications with escape sequences.

The reason why the pattern does not work is that JavaScript does not support lookbehinds ((?<=...). So you have to find a workaround for that. You won’t get around including that character in your pattern:

var pattern = /(?:^|\s):d(?!\S)/g;

Since there is no use in capturing anything in your pattern anyway (because :d is fixed) you are probably only interested in the position of the match. That means, when you find a match, you will have to check whether the first character is a space character (or is not :). If that is the case you have to increment the position by 1. If you know that your input string can never start with a space, you can simply increment any found position if it is not 0.

Note that I simplified your lookahead a bit. That is actually the beauty of lookarounds that you do not have to distinguish between end-of-string and a certain character type. Just use the negative lookahead, and assure that there is no non-space character ahead.

Just for future reference that means you could have simplified your initial pattern to:

(?<!\S):d(?!\S)

(If you were using a regex engine that supports lookbehinds.)

EDIT:

After your comment on the other answer, it’s actually a lot easier to use the workaround. Just write back the captured space-character:

string = string.replace(/(^|\s):d(?!\S)/g, "$1emoticonCode");

Where $1 refers to what was matched with (^|\s). I.e. if the match was at the beginning of the string $1 will be empty, and if there was a space before :d, then $1 will contian that space character.