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q with Perl Regex

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# 8  
Old 07-24-2008
Quote:
Originally Posted by KevinADC
\1 is a backreference to what is matched in the parenthesis in the regexp. So /(.)\1/ finds a double occurance of whatever (.) matched. It is similar to $1 but is used inside the regexp. It is discussed in some detail here:

perlretut - perldoc.perl.org
Ok I misunderstood. Thanks for bringing it up. Removed the incorrect paragraph from the original post to avoid distracting others viewing the thread.

It's good that there is someone kicking at your back and says "you're wrong". Isn't it? Smilie
Last edited by cbkihong; 07-24-2008 at 02:43 AM..
# 9  
Old 07-24-2008
Quote:
Originally Posted by fpmurphy
Interesting and thoughtful question. You use "(" and ")" to mark (remember) a pattern and recall the remembered pattern with "\" followed by a single digit (back reference).

In your particular case, "(.)\1" means remember a character and recall the character.

You can extend this method to find words with multiple double letters. '(.)\1(.)\2(.)\3' will match any word with three double letters, e.g. bookkeeper.
The logic can be extended further to find "double" words, like cancan and booboo:

Code:
/\b(\w\w\w)\1\b/

or repeated words, like "the the":
Code:
/\b(\w\w\w)\s\1\b/

and so on and so on...... (three repeated words) Smilie
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