http://snippets.dzone.com/posts Sun, 18 May 2008 03:47:22 GMT DZone Snippets: lookbehind code http://snippets.dzone.com/posts/show/5108 What follows below is the output from an irb session to specifically try out Lookahead and Lookbehind in regex. <br /> <br /><code> <br />irb(main):002:0> "question"[/q(?=u)/] <br />=> "q" <br />irb(main):003:0> "qu>"[/qu(?=>)/] <br />=> "qu" <br />irb(main):004:0> "qu"[/qu(?=>)/] <br />=> nil <br />irb(main):006:0> "accumulator"[/(?=c)cumulator/] <br />=> "cumulator" <br />irb(main):007:0> "accumulator"[/a(?=c)cumulator/] <br />=> nil <br />irb(main):009:0> "abc"[/abc(?=c)/] <br />=> nil <br />irb(main):010:0> "abc"[/ab(?=c)/] <br />=> "ab" <br /> <br />irb(main):011:0> "abd"[/ab(?!c)/] <br />=> "ab" <br />irb(main):012:0> "abc"[/ab(?!c)/] <br />=> nil <br /> <br />irb(main):013:0> "abc"[/a(?<=b)c/] # lookbehind doesn't work in Ruby <br />SyntaxError: compile error <br />(irb):13: undefined (?...) sequence: /a(?<=b)c/ <br /> from (irb):13 <br /> from :0 <br />irb(main):014:0> "abc"[/(?<=a)bc/] # lookbehind doesn't work in Ruby <br />SyntaxError: compile error <br />(irb):14: undefined (?...) sequence: /(?<=a)bc/ <br /> from (irb):14 <br /> from :0 <br /> <br />irb(main):022:0> "how's the weather?"[/how.*(?=the)/] <br />=> "how's the wea" <br />irb(main):024:0> "how's the weather?"[/how.*(?=[the])/] <br />=> "how's the weath" <br />irb(main):025:0> "how's the weather?"[/how.*(?=t)/] <br />=> "how's the wea" <br />irb(main):037:0> "how's the weather?"[/how.+(?=\bt)/] <br />=> "how's " <br /> <br />irb(main):057:0> "how's the weather?"[/ho.*(?=[a-z])/] <br />=> "how's the weathe" <br />irb(main):061:0> "how's the weather?"[/ho.*(?=['])/] <br />=> "how" <br /></code> <br />reference: <a href="http://www.regular-expressions.info/quickstart.html">Regular Expressions Quick Start</a> [regular-expressions.info] Tue, 05 Feb 2008 16:56:39 GMT http://snippets.dzone.com/posts/show/5108 jrobertson (James Robertson)