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Regarding Regular Expressions

Raj chiru
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Joined: Aug 12, 2008
Posts: 141
Hi Friends,

can you please explain What is differences among Greedy,Reluctant, and Possesive Quantifiers with example?
Actually I'm confusing with these Quantifiers.
Campbell Ritchie
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Joined: Oct 13, 2005
Posts: 39393
    
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Does this Java™ Tutorials link help?
Raj chiru
Ranch Hand

Joined: Aug 12, 2008
Posts: 141
Hi campbell,
Thanks For Your Reply,


Enter your regex: .*+foo // possessive quantifier
Enter input string to search: xfooxxxxxxfoo
No match found.


why we got No match found here?
Henry Wong
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Joined: Sep 28, 2004
Posts: 18896
    
  40

raj chiru wrote:
Enter your regex: .*+foo // possessive quantifier
Enter input string to search: xfooxxxxxxfoo
No match found.


why we got No match found here?


Because of the way the possessive modifier works. The possessive modifier acts like the greedy modifier, except that it doesn't back off. So...

The ".*+" portion will match everything, just like a greedy modifier. However, if it does that, while the ".*+" portion will match, the overall match will fail since there isn't anything left for the "foo" portion of the regex to match. In the case of the greedy modifier, it will backtrack, and match less, in order to allow the overall match to work.

Unfortunately, in the case of the possessive modifier, there isn't a backtrack, and hence, the overall match will fail. This is why you have to be really careful with the posessive modifier -- only use it for cases where you can perform the match correctly without a backtrack, because it won't.

Henry

Books: Java Threads, 3rd Edition, Jini in a Nutshell, and Java Gems (contributor)
 
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subject: Regarding Regular Expressions