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I have the following regular expression:

/(?<={index:)\d+(?=})/g

I am trying to find index integer in strings like this one:

some text{index:1}{id:2}{value:3}

That expression works fine with php, but it doesn't work in javascript, I get the following error:

Uncaught SyntaxError: Invalid regular expression: /(?<={index:)\d+(?=})/: Invalid group

What do I need to fix?

Thanks.

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  • 1
    try escaping your curly-brackets. Commented Nov 16, 2010 at 23:44
  • @jnpcl I just tried it a minute ago Uncaught SyntaxError: Invalid regular expression: /(?<=\{index:)\d+(?=\})/: Invalid group it doesn't work, unless there is another way to escape curly brackets other than \{ Commented Nov 16, 2010 at 23:47

3 Answers 3

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59

(?<= ) is a positive lookbehind. JavaScript's flavor of RegEx does not support lookbehinds (but it does support lookaheads).

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3 Comments

Actually, looks like lookbehinds are officially part of the JS Regex spec (developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…), but only supported in Chromium as of posting (developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…)
How can it be replaced and get the same behavior?
If nothing else works, you could reverse the string, do a reversed regexp match with a positive lookahead and reverse the result. But that may not apply to your problem you're having.
11

JavaScript does not support look-behind assertions. Use this pattern instead:

/{index:(\d+)}/g

Then extract the value captured in the group.

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3 Comments

it extracts the whole thing, in my case I just need an integer.
@negative: Notice the parentheses around the \d+; the integer is captured in group #1.
@Alan Moore, may be I am doing something wrong, but "some text{index:1}{id:2}{value:3}".match(/{index:(\d+)}/g)[0] returns {index:1} AND "some text{index:1}{id:2}{value:3}".match(/{index:(\d+)}/g)[1] returns 'undefined'.
5 
var str = "some text{index:1}{id:2}{value:3}";
var index = str.match(/{index:(\d+)}/);
index = index && index[1]*1;
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Ah, I see, no /g really helps =)

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