Regex from Learning Perl

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Regex from Learning Perl by CountZero (Bishop) on Feb 17, 2012 at 20:17 UTC
It matches: a literal "y" any single character not being a newline (save this in `\1`), in this case an 'a' is matched any single character not being a newline (save this in `\2`), in this case a 'b' is matched Match whatever is in `\2`, i.e. a 'b' Match whatever is in `\1`, i.e. an 'a' The match succeeds on `yabba` already. CountZero A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James My blog: Imperial Deltronics	[reply] [d/l] [select]
Re^2: Regex from Learning Perl by rayh (Initiate) on Feb 17, 2012 at 20:21 UTC
Awesome. Thanks!	[reply]
Re: Regex from Learning Perl by toolic (Bishop) on Feb 17, 2012 at 20:21 UTC
Basic debugging checklist, Tip #9: Demystify regular expressions by installing and using the CPAN module `YAPE::Regex::Explain` The regular expression: (?-imsx:y(.)(.)\2\1) matches as follows: NODE EXPLANATION ---------------------------------------------------------------------- (?-imsx: group, but do not capture (case-sensitive) (with ^ and $ matching normally) (with . not matching \n) (matching whitespace and # normally): ---------------------------------------------------------------------- y 'y' ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- . any character except \n ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- ( group and capture to \2: ---------------------------------------------------------------------- . any character except \n ---------------------------------------------------------------------- ) end of \2 ---------------------------------------------------------------------- \2 what was matched by capture \2 ---------------------------------------------------------------------- \1 what was matched by capture \1 ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- [download]	[reply] [d/l] [select]
Re: Regex from Learning Perl by Anonymous Monk on Feb 17, 2012 at 20:21 UTC
First (.) matches "a" and stores it in $1 Second (.) matches "b" and stores it in $2 \2 matches the value of $2 , it matches "b" \1 matches the value of $1 , it matches "a" `use re 'debug';` $ perl -Mre=debug -le " q/yabba dabba doo/ =~ /y(.)(.)\2\1/ " Compiling REx "y(.)(.)\2\1" Final program: 1: EXACT <y> (3) 3: OPEN1 (5) 5: REG_ANY (6) 6: CLOSE1 (8) 8: OPEN2 (10) 10: REG_ANY (11) 11: CLOSE2 (13) 13: REF2 (15) 15: REF1 (17) 17: END (0) anchored "y" at 0 (checking anchored) minlen 3 Guessing start of match in sv for REx "y(.)(.)\2\1" against "yabba dab +ba doo" Found anchored substr "y" at offset 0... Guessed: match at offset 0 Matching REx "y(.)(.)\2\1" against "yabba dabba doo" 0 <> <yabba dabb> \| 1:EXACT <y>(3) 1 <y> <abba dabba> \| 3:OPEN1(5) 1 <y> <abba dabba> \| 5:REG_ANY(6) 2 <ya> <bba dabba > \| 6:CLOSE1(8) 2 <ya> <bba dabba > \| 8:OPEN2(10) 2 <ya> <bba dabba > \| 10:REG_ANY(11) 3 <yab> <ba dabba d> \| 11:CLOSE2(13) 3 <yab> <ba dabba d> \| 13:REF2(15) 4 <yabb> <a dabba do> \| 15:REF1(17) 5 <yabba> < dabba doo> \| 17:END(0) Match successful! Freeing REx: "y(.)(.)\2\1" [download] YAPE::Regex::Explain use YAPE::Regex::Explain; print YAPE::Regex::Explain->new( qr/y(.)(.)\2\1/ )->explain; __END__ The regular expression: (?-imsx:y(.)(.)\2\1) matches as follows: NODE EXPLANATION ---------------------------------------------------------------------- (?-imsx: group, but do not capture (case-sensitive) (with ^ and $ matching normally) (with . not matching \n) (matching whitespace and # normally): ---------------------------------------------------------------------- y 'y' ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- . any character except \n ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- ( group and capture to \2: ---------------------------------------------------------------------- . any character except \n ---------------------------------------------------------------------- ) end of \2 ---------------------------------------------------------------------- \2 what was matched by capture \2 ---------------------------------------------------------------------- \1 what was matched by capture \1 ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- [download]	[reply] [d/l] [select]
Re^2: Regex from Learning Perl by Anonymous Monk on Feb 17, 2012 at 20:28 UTC
$ perl -Mre=debug -le " @F = q/yabba yobbo yebbe yubbu/ =~ /y(.)(.)\2\ +1/g " Compiling REx "y(.)(.)\2\1" Final program: 1: EXACT <y> (3) 3: OPEN1 (5) 5: REG_ANY (6) 6: CLOSE1 (8) 8: OPEN2 (10) 10: REG_ANY (11) 11: CLOSE2 (13) 13: REF2 (15) 15: REF1 (17) 17: END (0) anchored "y" at 0 (checking anchored) minlen 3 Guessing start of match in sv for REx "y(.)(.)\2\1" against "yabba yob +bo yebbe yubbu" Found anchored substr "y" at offset 0... Guessed: match at offset 0 Matching REx "y(.)(.)\2\1" against "yabba yobbo yebbe yubbu" 0 <> <yabba yobb> \| 1:EXACT <y>(3) 1 <y> <abba yobbo> \| 3:OPEN1(5) 1 <y> <abba yobbo> \| 5:REG_ANY(6) 2 <ya> <bba yobbo > \| 6:CLOSE1(8) 2 <ya> <bba yobbo > \| 8:OPEN2(10) 2 <ya> <bba yobbo > \| 10:REG_ANY(11) 3 <yab> <ba yobbo y> \| 11:CLOSE2(13) 3 <yab> <ba yobbo y> \| 13:REF2(15) 4 <yabb> <a yobbo ye> \| 15:REF1(17) 5 <yabba> < yobbo yeb> \| 17:END(0) Match successful! Guessing start of match in sv for REx "y(.)(.)\2\1" against " yobbo ye +bbe yubbu" Found anchored substr "y" at offset 1... Starting position does not contradict /^/m... Guessed: match at offset 1 Matching REx "y(.)(.)\2\1" against "yobbo yebbe yubbu" 6 <abba > <yobbo yebb> \| 1:EXACT <y>(3) 7 <bba y> <obbo yebbe> \| 3:OPEN1(5) 7 <bba y> <obbo yebbe> \| 5:REG_ANY(6) 8 <ba yo> <bbo yebbe > \| 6:CLOSE1(8) 8 <ba yo> <bbo yebbe > \| 8:OPEN2(10) 8 <ba yo> <bbo yebbe > \| 10:REG_ANY(11) 9 <a yob> <bo yebbe y> \| 11:CLOSE2(13) 9 <a yob> <bo yebbe y> \| 13:REF2(15) 10 < yobb> <o yebbe yu> \| 15:REF1(17) 11 <yobbo> < yebbe yub> \| 17:END(0) Match successful! Guessing start of match in sv for REx "y(.)(.)\2\1" against " yebbe yu +bbu" Found anchored substr "y" at offset 1... Starting position does not contradict /^/m... Guessed: match at offset 1 Matching REx "y(.)(.)\2\1" against "yebbe yubbu" 12 <obbo > <yebbe yubb> \| 1:EXACT <y>(3) 13 <bbo y> <ebbe yubbu> \| 3:OPEN1(5) 13 <bbo y> <ebbe yubbu> \| 5:REG_ANY(6) 14 <bo ye> <bbe yubbu> \| 6:CLOSE1(8) 14 <bo ye> <bbe yubbu> \| 8:OPEN2(10) 14 <bo ye> <bbe yubbu> \| 10:REG_ANY(11) 15 <o yeb> <be yubbu> \| 11:CLOSE2(13) 15 <o yeb> <be yubbu> \| 13:REF2(15) 16 < yebb> <e yubbu> \| 15:REF1(17) 17 < yebbe> < yubbu> \| 17:END(0) Match successful! Guessing start of match in sv for REx "y(.)(.)\2\1" against " yubbu" Found anchored substr "y" at offset 1... Starting position does not contradict /^/m... Guessed: match at offset 1 Matching REx "y(.)(.)\2\1" against "yubbu" 18 < yebbe > <yubbu> \| 1:EXACT <y>(3) 19 < yebbe y> <ubbu> \| 3:OPEN1(5) 19 < yebbe y> <ubbu> \| 5:REG_ANY(6) 20 < yebbe yu> <bbu> \| 6:CLOSE1(8) 20 < yebbe yu> <bbu> \| 8:OPEN2(10) 20 < yebbe yu> <bbu> \| 10:REG_ANY(11) 21 < yebbe yub> <bu> \| 11:CLOSE2(13) 21 < yebbe yub> <bu> \| 13:REF2(15) 22 < yebbe yubb> <u> \| 15:REF1(17) 23 < yebbe yubbu> <> \| 17:END(0) Match successful! Freeing REx: "y(.)(.)\2\1" [download]	[reply] [d/l]


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