(a|b)\1
What does \1 mean in this expression?
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Tim Pietzcker
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bitmapdata.com
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2 Answers
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\1 - it means the first capturing group in the matched expression. \n would be the nth capturing group. (Note that \0 would be whole match). In many engines, the upperlimit for n is 9, but some support up to 99 as well.
When used in regex like (a|b)\1, it means that after a or b, the next character should be the first captured group, which is a or b so the regex here would match aa or bb.
manojlds
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\2 it means the second group in the match expression? right? \3...\9 means the third...nineth group in the match expression? – bitmapdata.com Dec 24 '11 at 11:51
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@bitmapdata.com - Yes. If there is one. – manojlds Dec 24 '11 at 11:52
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@bitmapdata.com - You too. You can accept as answer if it helped you :) – manojlds Dec 24 '11 at 11:58
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@TimPietzcker - BF moment. Corrected. – manojlds Dec 24 '11 at 12:03
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If refers to what was matched in the first set of parentheses, the first group. Subsequent number means subsequent parentheses.
(1|2)(3|4)\1\2
Would match:
1313
1414
2323
2424
Not that if you have nested groups, just count from the opening brace (left brace).
(groupOne(groupTwo)stillOne(groupThree(groupFour)))
sidyll
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Thanks @bitmapdata.com ! Merry christmas and a happy and wonderful 2012! – sidyll Dec 24 '11 at 12:06