Why doesn't this RegEx work in JavaScript?

Question

I'm trying to match the text "foo" between the two brackets in this following statement:

Expected [foo] but Recieved [__]

I'm then trying to replace the text with "...". When I run the following code I get an invalid grouping error in JavaScript. When I run the expression in Regexr it works the way I expect it to.

var text = $("#assertion").html().replace( /(?<=Expected \[).*(?=\] )/,"...");

http://jsfiddle.net/bittersweetryan/VcraW/

You can solve this problem by not using lookbacks. Just include the lookback text in the regex as regular text to match with parens around it and then use $1 in your replacement string to put it back into the replacement. — jfriend00
– jfriend00, Commented Nov 25, 2011 at 16:04

hugomg · Accepted Answer · 2011-11-25 16:13:47Z

6

Javascript regexes don't understand lookbacks (the ?<=). You need to match the Expected [ part explicitely:

replace(/(Expected \[)[^\]]*/, "$1...")

The $1 is to avoid retyping the "Expected [" part and I changed the rest of the regex a bit to avoid greedy matching with .*

http://jsfiddle.net/VcraW/2/

edited Nov 25, 2011 at 16:13

answered Nov 25, 2011 at 15:59

hugomg

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1 Comment

stema Over a year ago

+1 for the 25 seconds you were faster and having exactly the same thoughts than me.

stema · Accepted Answer · 2011-11-25 15:59:46Z

3

Because javascript does not support look behind assertions.

You can work around by putting this in a capturing group and using this in the replacement

.replace( /(Expected \[).*(?=\] )/,"$1...");

The content of the first pair of brackets is stored in the capturing group 1 and you can get this back using $1

answered Nov 25, 2011 at 15:59

stema

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