homepage Welcome to WebmasterWorld Guest from 54.196.189.229
register, free tools, login, search, subscribe, help, library, announcements, recent posts, open posts,
Subscribe to WebmasterWorld

Home / Forums Index / Code, Content, and Presentation / Perl Server Side CGI Scripting
Forum Library, Charter, Moderators: coopster & jatar k & phranque

Perl Server Side CGI Scripting Forum

    
Regex
Davo1977




msg:3789989
 10:09 pm on Nov 19, 2008 (gmt 0)

I am really struggling to come to terms with look-ahead and look-behind concepts in Regular Expressions.

I am new to Perl and can only start to get an understanding with maybe an example that explains how these concepts work.

Can anyone help me with this?

 

ergophobe




msg:3791540
 6:13 pm on Nov 21, 2008 (gmt 0)

The lookahead is a 'zero width' expression so it allows you to see if you have a match, without having to capture the characters you're matching. So for example

a[^b] - matches "a" and whatever character follows an "a" as long as it's not a "b"

a(?!b) - matches "a" and only "a" provided that it is not followed by a "b" but it does not match the character after the "a".

So for example in "example" the first expression would capture "am" and the second would capture "a".

I'm trying to think of a case where you can't do it any other way, but I'm not coming up with one. You'll see, sometimes, that a negated character class won't get the job done though.

Example anyone?

Global Options:
 top home search open messages active posts  
 

Home / Forums Index / Code, Content, and Presentation / Perl Server Side CGI Scripting
rss feed

All trademarks and copyrights held by respective owners. Member comments are owned by the poster.
Home ¦ Free Tools ¦ Terms of Service ¦ Privacy Policy ¦ Report Problem ¦ About ¦ Library ¦ Newsletter
WebmasterWorld is a Developer Shed Community owned by Jim Boykin.
© Webmaster World 1996-2014 all rights reserved