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Regexp help -- (a*)(b*|(ab)*) and (a*)((b|ab)*) on "aabab"

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Top Forums Shell Programming and Scripting Regexp help -- (a*)(b*|(ab)*) and (a*)((b|ab)*) on "aabab"
# 1  
Old 03-23-2009
Regexp help -- (a*)(b*|(ab)*) and (a*)((b|ab)*) on "aabab"
Hi all

A regular expression question -- (a*)(b*|(ab)*) and (a*)((b|ab)*) on "aabab"

First, (a*)(b*|(ab)*) on "aabab". I though it would match "aa-b". But it turns on "a-abab". So why isn't a* greedy here?

Then I try (a*)((b|ab)*). It also matches aa-bab. But this time a* is greedy here. Why ?
# 2  
Old 03-23-2009
It depends, different regular expressions engines work differently:

Code:
% perl -ple's/(a*)(b*|(ab)*)/-->$1<-- -->$2<--/'<<<aabab
-->aa<-- -->b<--ab
% sed -r 's/(a*)(b*|(ab)*)/-->\1<-- -->\2<--/'<<<aabab  
-->a<-- -->abab<--
% awk '{print gensub(/(a*)(b*|(ab)*)/,"-->\\1<-- -->\\2<--",1)}'<<<aabab
-->a<-- -->abab<--

# 3  
Old 03-23-2009
TCL 8.5 also works in the way I described
Thanks for the reply.

TCL 8.5 also works in the way I described.

Which version of perl you use ?
# 4  
Old 03-23-2009
v5.10.0 built for cygwin-thread-multi-64int for this example.

Tried also 5.005_03 built for sun4-solaris and v5.8.8 built for IA64.ARCHREV_0-thread-multi with the same result.

And, of course, the Perl regular expressions engine is quite powerful, consider the following:

Code:
$ perl -pe's/(a*?)(b*|(ab)*)/-->$1<-- -->$2<--/'<<<aabab
--><-- --><--aabab # nothing matches, 0 or more non-greedy, so 0.
$ perl -pe's/(a+?)(b*|(ab)*)/-->$1<-- -->$2<--/'<<<aabab
-->a<-- --><--abab # one or more, non-greedy

Last edited by radoulov; 03-23-2009 at 01:09 PM..
# 5  
Old 03-23-2009
Did you also try (a*)((b|ab)*) to see if the difference exists ?
# 6  
Old 03-23-2009
Only the second backreference is different:

Code:
% perl -pe's/(a*)((b|ab)*)/-->$1<-- -->$2<--/'<<<aabab 
-->aa<-- -->bab<--

Last edited by radoulov; 03-23-2009 at 01:27 PM.. Reason: corrected
# 7  
Old 03-23-2009
This might be helpful:

Code:
% perl -M're debug' -pe'/(a*)(b*|(ab)*)/'<<<aabab
Compiling REx "(a*)(b*|(ab)*)"
Final program:
   1: OPEN1 (3)
   3:   STAR (6)
   4:     EXACT <a> (0)
   6: CLOSE1 (8)
   8: OPEN2 (10)
  10:   BRANCH (14)
  11:     STAR (25)
  12:       EXACT <b> (0)
  14:   BRANCH (FAIL)
  15:     CURLYM[3] {0,32767} (25)
  19:       EXACT <ab> (23)
  23:       SUCCEED (0)
  24:     NOTHING (25)
  25: CLOSE2 (27)
  27: END (0)
minlen 0 
Matching REx "(a*)(b*|(ab)*)" against "aabab%n"
   0 <> <aabab%n>            |  1:OPEN1(3)
   0 <> <aabab%n>            |  3:STAR(6)
                                  EXACT <a> can match 2 times out of 2147483647...
   2 <aa> <bab%n>            |  6:  CLOSE1(8)
   2 <aa> <bab%n>            |  8:  OPEN2(10)
   2 <aa> <bab%n>            | 10:  BRANCH(14)
   2 <aa> <bab%n>            | 11:    STAR(25)
                                      EXACT <b> can match 1 times out of 2147483647...
   3 <aab> <ab%n>            | 25:      CLOSE2(27)
   3 <aab> <ab%n>            | 27:      END(0)
Match successful!
aabab
Freeing REx: "(a*)(b*|(ab)*)"

Code:
% perl -M're debug' -pe'/(a*)((b|ab)*)/'<<<aabab      
Compiling REx "(a*)((b|ab)*)"
Final program:
   1: OPEN1 (3)
   3:   STAR (6)
   4:     EXACT <a> (0)
   6: CLOSE1 (8)
   8: OPEN2 (10)
  10:   CURLYX[1] {0,32767} (23)
  12:     OPEN3 (14)
  14:       TRIE-EXACT[ab] (20)
            <b> 
            <ab> 
  20:     CLOSE3 (22)
  22:   WHILEM[1/1] (0)
  23:   NOTHING (24)
  24: CLOSE2 (26)
  26: END (0)
minlen 0 
Matching REx "(a*)((b|ab)*)" against "aabab%n"
   0 <> <aabab%n>            |  1:OPEN1(3)
   0 <> <aabab%n>            |  3:STAR(6)
                                  EXACT <a> can match 2 times out of 2147483647...
   2 <aa> <bab%n>            |  6:  CLOSE1(8)
   2 <aa> <bab%n>            |  8:  OPEN2(10)
   2 <aa> <bab%n>            | 10:  CURLYX[1] {0,32767}(23)
   2 <aa> <bab%n>            | 22:    WHILEM[1/1](0)
                                      whilem: matched 0 out of 0..32767
   2 <aa> <bab%n>            | 12:      OPEN3(14)
   2 <aa> <bab%n>            | 14:      TRIE-EXACT[ab](20)
   2 <aa> <bab%n>            |          State:    1 Accepted:    0 Charid:  1 CP:  62 After State:    2
   3 <aab> <ab%n>            |          State:    2 Accepted:    1 Charid:  1 CP:   0 After State:    0
                                        got 1 possible matches
                                        only one match left: #1 <b>
   3 <aab> <ab%n>            | 20:      CLOSE3(22)
   3 <aab> <ab%n>            | 22:      WHILEM[1/1](0)
                                        whilem: matched 1 out of 0..32767
   3 <aab> <ab%n>            | 12:        OPEN3(14)
   3 <aab> <ab%n>            | 14:        TRIE-EXACT[ab](20)
   3 <aab> <ab%n>            |            State:    1 Accepted:    0 Charid:  2 CP:  61 After State:    3
   4 <aaba> <b%n>            |            State:    3 Accepted:    0 Charid:  1 CP:  62 After State:    4
   5 <aabab> <%n>            |            State:    4 Accepted:    1 Charid:  2 CP:   0 After State:    0
                                          got 1 possible matches
                                          only one match left: #2 <ab>
   5 <aabab> <%n>            | 20:        CLOSE3(22)
   5 <aabab> <%n>            | 22:        WHILEM[1/1](0)
                                          whilem: matched 2 out of 0..32767
   5 <aabab> <%n>            | 12:          OPEN3(14)
   5 <aabab> <%n>            | 14:          TRIE-EXACT[ab](20)
                                            failed to match trie start class...
                                          whilem: failed, trying continuation...
   5 <aabab> <%n>            | 23:          NOTHING(24)
   5 <aabab> <%n>            | 24:          CLOSE2(26)
   5 <aabab> <%n>            | 26:          END(0)
Match successful!
aabab
Freeing REx: "(a*)((b|ab)*)"

For more info on how to interpret the output check this or try:

Code:
perldoc perldebguts|less -p'regular'

Last edited by radoulov; 03-23-2009 at 01:50 PM..
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