http://snippets.dzone.com/posts Fri, 25 Jul 2008 13:43:40 GMT DZone Snippets: backreferences code http://snippets.dzone.com/posts/show/5096 This irb session example shows back referencing, where a token enclosed within round brackets is automatically assigned as a variable. That variable is represented by a dollar ($) symbol followed by it's index number starting at 1. <br /> <br /><code> <br />#match capture = STRONG <br />irb(main):652:0> "<STRONG>text here</STRONG>"[/<(.*)>.*<\/\1>/] <br />=> "<STRONG>text here</STRONG>" <br /> <br />irb(main):655:0> "abttba"[/^(.)(.).*\2\1$/] <br />=> "abttba" <br />irb(main):658:0> "ab123ttba"[/^(.)(.)(123).*\2\1\3$/] <br />=> nil <br />irb(main):659:0> "ab123ttba123"[/^(.)(.)(123).*\2\1\3$/] <br />=> "ab123ttba123" <br />irb(main):663:0> "how are you"[/(o)*\1/] <br />=> nil <br />irb(main):664:0> "how are you"[/(o).*\1/] <br />=> "ow are yo" <br />irb(main):685:0> "how are you"[/(o)[^\1]+\1/] <br />=> "ow are yo" <br />irb(main):686:0> "how are you"[/(o)[^\1]+\1u/] <br />=> "ow are you" <br />irb(main):692:0> "how is Lucy?"[/(o)[^\1]+\1/] <br />=> nil <br />irb(main):693:0> "how is Lucy?"[/(o)[^\1]+\1?/] <br />=> "ow is Lucy?" <br /> <br />irb(main):702:0> "I eat now?".gsub(/(.)\s(\w+)\s(\w+)(.)/,$1) <br />=> "I" <br />irb(main):708:0> "I eat now?".gsub(/(.)\s(\w+)\s(\w+)(.)/,"#{$3} #{$2}") <br />=> "now eat" <br />irb(main):711:0> "I eat now?".gsub(/(.)\s(\w+)\s(\w+)(.)/,"#{$3} #{$1} #{$2}#{$4}") <br />=> "now I eat?" <br /> <br />irb(main):712:0> $1 <br />=> "I" <br /></code> <br /> <br />Backreference support in Ruby uses the following variables: <br /> <br /> $` returns everything before the matched string. <br /> $' returns everything after the matched string. <br /> $+ returns whatever the last bracket match matched. <br /> $& returns the entire matched string. <br /> <br /><code> <br />irb(main):713:0> $& <br />=> "I eat now?" <br /> <br />irb(main):715:0> "I eat now? Mother?".gsub(/(.)\s(\w+)\s(\w+)(.)/,"#{$3} #{$1} #{$2}#{$4}") <br />=> "now I eat? Mother?" <br /> <br />irb(main):716:0> $' <br />=> " Mother?" <br />irb(main):721:0> $'[/M/] <br />=> "M" <br /> <br />irb(main):732:0> "I'm hungry ... I eat now? Mother?".gsub(/[^.](.)\s(\w+)\s(\w+)(.)/,"#{$3} #{$1} #{$2}#{$4}") <br />=> "I'm hungry ...now I eat? Mother?" <br />irb(main):733:0> $` <br />=> "I'm hungry ..." <br /></code> <br /> <br />Note: These Ruby examples should work from <a href="http://www.rubular.com/">Rubular</a> [rubular.com]. <br />Reference: <a href="http://www.webreference.com/js/column5/values.html">Unix Regular Expressions: Backreferences</a> [webreference.com] Sun, 03 Feb 2008 12:37:29 GMT http://snippets.dzone.com/posts/show/5096 jrobertson (James Robertson)