(a|b)\1
What does \1 mean in this expression?
Asked
Active
Viewed 3.0k times
50
Tim Pietzcker
- 328,213
- 58
- 503
- 561
bitmapdata.com
- 9,572
- 5
- 35
- 43
2 Answers
72
\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
- 290,304
- 63
- 469
- 417
-
\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
-
@bitmapdata.com - Yes. If there is one. – manojlds Dec 24 '11 at 11:52
-
1@bitmapdata.com - You too. You can accept as answer if it helped you :) – manojlds Dec 24 '11 at 11:58
-
@TimPietzcker - BF moment. Corrected. – manojlds Dec 24 '11 at 12:03
-
This feature is called backreference; A backreference in a regular expression identifies a previously matched group and looks for exactly the same text again. – SAMPro Mar 11 '22 at 08:32
28
It 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
Note that if you have nested groups, just count from the opening brace (left brace).
(groupOne(groupTwo)stillOne(groupThree(groupFour)))
Fabio says Reinstate Monica
- 5,271
- 9
- 40
- 61
sidyll
- 57,726
- 14
- 108
- 151
-
-
Thanks @bitmapdata.com ! Merry christmas and a happy and wonderful 2012! – sidyll Dec 24 '11 at 12:06