in reply to Regex code block executes twice per match using look-arounds

You can get some insight into how this happens in your code with use re 'debug'. I've altered the target string to {x1}[x2] for simplicity.

After several unsuccessful attempts, the engine finds a match starting at position 4: 

  Setting an EVAL scope, savestack=25
   4 <{x1}> <[x2]>        |  1:  IFMATCH[-1]
   3 <{x1> <}[x2]>        |  3:    OPEN1
   3 <{x1> <}[x2]>        |  5:    ANYOF[)>\]}]
   4 <{x1}> <[x2]>        | 16:    CLOSE1
   4 <{x1}> <[x2]>        | 18:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 20:  IFMATCH[-0]
   4 <{x1}> <[x2]>        | 22:    OPEN2
   4 <{x1}> <[x2]>        | 24:    ANYOF[(<[{]
   5 <{x1}[> <x2]>        | 35:    CLOSE2
   5 <{x1}[> <x2]>        | 37:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 39:  EVAL
  re_eval 0x10033008
Match 1: left }, right [
   4 <{x1}> <[x2]>        | 41:  END
Match successful!

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Then it starts again at position 4, and matches again but then throws it away:

  Setting an EVAL scope, savestack=37
   4 <{x1}> <[x2]>        |  1:  IFMATCH[-1]
   3 <{x1> <}[x2]>        |  3:    OPEN1
   3 <{x1> <}[x2]>        |  5:    ANYOF[)>\]}]
   4 <{x1}> <[x2]>        | 16:    CLOSE1
   4 <{x1}> <[x2]>        | 18:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 20:  IFMATCH[-0]
   4 <{x1}> <[x2]>        | 22:    OPEN2
   4 <{x1}> <[x2]>        | 24:    ANYOF[(<[{]
   5 <{x1}[> <x2]>        | 35:    CLOSE2
   5 <{x1}[> <x2]>        | 37:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 39:  EVAL
  re_eval 0x10033008
Match 2: left }, right [
   4 <{x1}> <[x2]>        | 41:  END
Match possible, but length=0 is smaller than requested=1, failing!

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The whole output looks like this: 
Compiling REx `[]>})]'
size 12 Got 100 bytes for offset annotations.
first at 1
   1: ANYOF[)>\]}](12)
  12: END(0)
stclass `ANYOF[)>\]}]' minlen 1 
Offsets: [12]
    1[6] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 7[0] 
Compiling REx `[[<{(]'
size 12 Got 100 bytes for offset annotations.
first at 1
   1: ANYOF[(<[{](12)
  12: END(0)
stclass `ANYOF[(<[{]' minlen 1 
Offsets: [12]
    1[6] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 7[0] 
Compiling REx `
   (?<=((?-xism:[]>})])))
   (?=((?-xism:[[<{(])))
   (?{print qq{Match @{ [++ $count] }: left $1, right $2\n}})
'
size 41 Got 332 bytes for offset annotations.
first at 1
   1: IFMATCH[-1](20)
   3:   OPEN1(5)
   5:     ANYOF[)>\]}](16)
  16:   CLOSE1(18)
  18:   SUCCEED(0)
  19:   TAIL(20)
  20: IFMATCH[-0](39)
  22:   OPEN2(24)
  24:     ANYOF[(<[{](35)
  35:   CLOSE2(37)
  37:   SUCCEED(0)
  38:   TAIL(39)
  39: EVAL(41)
  41: END(0)
minlen 0 with eval 
Offsets: [41]
    9[17] 0[0] 9[1] 0[0] 18[6] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0]
+ 0[0] 0[0] 25[1] 0[0] 25[0] 25[0] 34[17] 0[0] 34[1] 0[0] 43[6] 0[0] 0
+[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 50[1] 0[0] 50[0] 50[0] 56
+[59] 0[0] 115[0] 
 Before: {x1}[x2]
----------------------------------------
Matching REx `
   (?<=((?-xism:[]>})])))
   (?=((?-xism:[[<{(])))
   (?{print qq{Match @{ [++ $count] }: left $1, right $2\n}})
...' against `{x1}[x2]'
  Setting an EVAL scope, savestack=25
   0 <> <{x1}[x2]>        |  1:  IFMATCH[-1]
                            failed...
  Setting an EVAL scope, savestack=25
   1 <{> <x1}[x2]>        |  1:  IFMATCH[-1]
   0 <> <{x1}[x2]>        |  3:    OPEN1
   0 <> <{x1}[x2]>        |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=25
   2 <{x> <1}[x2]>        |  1:  IFMATCH[-1]
   1 <{> <x1}[x2]>        |  3:    OPEN1
   1 <{> <x1}[x2]>        |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=25
   3 <{x1> <}[x2]>        |  1:  IFMATCH[-1]
   2 <{x> <1}[x2]>        |  3:    OPEN1
   2 <{x> <1}[x2]>        |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=25
   4 <{x1}> <[x2]>        |  1:  IFMATCH[-1]
   3 <{x1> <}[x2]>        |  3:    OPEN1
   3 <{x1> <}[x2]>        |  5:    ANYOF[)>\]}]
   4 <{x1}> <[x2]>        | 16:    CLOSE1
   4 <{x1}> <[x2]>        | 18:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 20:  IFMATCH[-0]
   4 <{x1}> <[x2]>        | 22:    OPEN2
   4 <{x1}> <[x2]>        | 24:    ANYOF[(<[{]
   5 <{x1}[> <x2]>        | 35:    CLOSE2
   5 <{x1}[> <x2]>        | 37:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 39:  EVAL
  re_eval 0x10033008
Match 1: left }, right [
   4 <{x1}> <[x2]>        | 41:  END
Match successful!
Matching REx `
   (?<=((?-xism:[]>})])))
   (?=((?-xism:[[<{(])))
   (?{print qq{Match @{ [++ $count] }: left $1, right $2\n}})
...' against `[x2]'
  Setting an EVAL scope, savestack=37
   4 <{x1}> <[x2]>        |  1:  IFMATCH[-1]
   3 <{x1> <}[x2]>        |  3:    OPEN1
   3 <{x1> <}[x2]>        |  5:    ANYOF[)>\]}]
   4 <{x1}> <[x2]>        | 16:    CLOSE1
   4 <{x1}> <[x2]>        | 18:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 20:  IFMATCH[-0]
   4 <{x1}> <[x2]>        | 22:    OPEN2
   4 <{x1}> <[x2]>        | 24:    ANYOF[(<[{]
   5 <{x1}[> <x2]>        | 35:    CLOSE2
   5 <{x1}[> <x2]>        | 37:    SUCCEED
                              could match...
   4 <{x1}> <[x2]>        | 39:  EVAL
  re_eval 0x10033008
Match 2: left }, right [
   4 <{x1}> <[x2]>        | 41:  END
Match possible, but length=0 is smaller than requested=1, failing!
  Clearing an EVAL scope, savestack=37..40
  Setting an EVAL scope, savestack=37
   5 <{x1}[> <x2]>        |  1:  IFMATCH[-1]
   4 <{x1}> <[x2]>        |  3:    OPEN1
   4 <{x1}> <[x2]>        |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=37
   6 <{x1}[x> <2]>        |  1:  IFMATCH[-1]
   5 <{x1}[> <x2]>        |  3:    OPEN1
   5 <{x1}[> <x2]>        |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=37
   7 <{x1}[x2> <]>        |  1:  IFMATCH[-1]
   6 <{x1}[x> <2]>        |  3:    OPEN1
   6 <{x1}[x> <2]>        |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=37
   8 <{x1}[x2]> <>        |  1:  IFMATCH[-1]
   7 <{x1}[x2> <]>        |  3:    OPEN1
   7 <{x1}[x2> <]>        |  5:    ANYOF[)>\]}]
   8 <{x1}[x2]> <>        | 16:    CLOSE1
   8 <{x1}[x2]> <>        | 18:    SUCCEED
                              could match...
   8 <{x1}[x2]> <>        | 20:  IFMATCH[-0]
   8 <{x1}[x2]> <>        | 22:    OPEN2
   8 <{x1}[x2]> <>        | 24:    ANYOF[(<[{]
                              failed...
                            failed...
Match failed
----------------------------------------
  After: {x1}+[x2]
Freeing REx: `"[]>})]"'
Freeing REx: `"[[<{(]"'
Freeing REx: `"\n   (?<=((?-xism:[]>})])))\n   (?=((?-xism:[[<{(])))\n
+   ("......'

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Replies are listed 'Best First'.
Re^2: Regex code block executes twice per match using look-arounds
by johngg (Canon) on Jul 12, 2007 at 19:39 UTC
    Thank you for your reply. As you can see from my reply to ikegami we have been thinking in parallel. I have tried to get my head around the output from re debug before but found the learning curve very steep. Today in the documentation I spotted use re 'debugcolor'; which gave me enough of a helping hand (even in black and white) to start to penetrate the mystery. I have been meaning to get the Owl Book mentioned by sgt for some time but today has shown that I have a long way to go in my understanding; buying the book is now a priority.

    Cheers,

    JohnGG

[reply]
[d/l]
      Yeah, it's cryptic. There's some info on interpreting it in perldebguts .
[reply]

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