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Perl Regex: not processing back references correctly

rajkumar
New User

Oct 9, 2012, 11:05 PM

Post #1 of 2 (5998 views)

Perl Regex: not processing back references correctly	Can't Post

Hi,

According to: http://www.boost.org/doc/libs/1_33_1/libs/regex/doc/syntax_perl.html

^(a*).*\1$

should not match

aaabba

But I have checked out many perl online testing site and perl script also, all matches above string correctly.

I am confused now , which one is correct 1. Boost documentation 2. Perl script.
Please let me know which one is correct.

I am not able to understand ^(a*).*\1$ regex also. Please assist me to understand this also.

Thanks in advance.

Laurent_R
Enthusiast / Moderator

Oct 10, 2012, 4:49 AM

Post #2 of 2 (5993 views)

Re: [rajkumar] Perl Regex: not processing back references correctly [In reply to]	Can't Post

Hi,

I see no reason why ^(a*).*\1$ should fail to match aaabba.

And I confirm that it DOES indeed match it.

The regex engine first tries "aaa", but fails on the back reference. It then backtracks and tries "aa", and fails again. It backtracks again and tries "a", and then succeeds.

Example on the command line:

Code

perl -e '$c = "aaabba"; print "matched \"$1\"\n" if  $c =~ /^(a*).*\1$/;' 
matched "a"

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