comment on

advance pointer the length of the match

That was the fundamental piece of the puzzle that I was missing. Indeed, after jettero's 2nd reply I started to investigate with use re 'debug'; and could see the mechanism you describe.

I changed the regex so that it was using the look-behind but the look-ahead was replaced with a simple capture

...
my $rxBetween = qr
   {(?x)
       (?<=($rxClose))
       ($rxOpen)
       (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}}
+)
   };
...
$string =~ s{$rxBetween}{+$2}g;
...
[download]

and that stopped the double execution. It also seemed clear from the debug output that using both look-arounds was making the engine do a lot more work.

Using look-behind and look-ahead

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 `(?x)
       (?<=((?-xism:[]>})])))
       (?=((?-xism:[[<{(])))
       (?{print qq{Match @{ [++ $count] }: on left $1, on 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]
        17[17] 0[0] 17[1] 0[0] 26[6] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0
+] 0[0] 0[0] 0[0] 33[1] 0[0] 33[0] 33[0] 46[17] 0[0] 46[1] 0[0] 55[6] 
+0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 62[1] 0[0] 62[0] 62
+[0] 72[68] 0[0] 140[0] 
 String: <x1>[x2]{x3}(x4)
----------------------------------------
Matching REx `(?x)
       (?<=((?-xism:[]>})])))
       (?=((?-xism:[[<{(])))
       (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}}
+)
   ...' against `<x1>[x2]{x3}(x4)'
  Setting an EVAL scope, savestack=25
   0 <> <<x1>[x2]{x3}>    |  1:  IFMATCH[-1]
                            failed...
  Setting an EVAL scope, savestack=25
   1 <<> <x1>[x2]{x3}>    |  1:  IFMATCH[-1]
   0 <> <<x1>[x2]{x3}>    |  3:    OPEN1
   0 <> <<x1>[x2]{x3}>    |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=25
   2 <<x> <1>[x2]{x3}>    |  1:  IFMATCH[-1]
   1 <<> <x1>[x2]{x3}>    |  3:    OPEN1
   1 <<> <x1>[x2]{x3}>    |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=25
   3 <<x1> <>[x2]{x3}>    |  1:  IFMATCH[-1]
   2 <<x> <1>[x2]{x3}>    |  3:    OPEN1
   2 <<x> <1>[x2]{x3}>    |  5:    ANYOF[)>\]}]
                              failed...
                            failed...
  Setting an EVAL scope, savestack=25
   4 <<x1>> <[x2]{x3}>    |  1:  IFMATCH[-1]
   3 <<x1> <>[x2]{x3}>    |  3:    OPEN1
   3 <<x1> <>[x2]{x3}>    |  5:    ANYOF[)>\]}]
   4 <<x1>> <[x2]{x3}>    | 16:    CLOSE1
   4 <<x1>> <[x2]{x3}>    | 18:    SUCCEED
                              could match...
   4 <<x1>> <[x2]{x3}>    | 20:  IFMATCH[-0]
   4 <<x1>> <[x2]{x3}>    | 22:    OPEN2
   4 <<x1>> <[x2]{x3}>    | 24:    ANYOF[(<[{]
   5 <<x1>[> <x2]{x3}>    | 35:    CLOSE2
   5 <<x1>[> <x2]{x3}>    | 37:    SUCCEED
                              could match...
   4 <<x1>> <[x2]{x3}>    | 39:  EVAL
  re_eval 0x63a28
Match 1: on left >, on right [
   4 <<x1>> <[x2]{x3}>    | 41:  END
Match successful!
Matching REx `(?x)
       (?<=((?-xism:[]>})])))
       (?=((?-xism:[[<{(])))
       (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}}
+)
   ...' against `[x2]{x3}(x4)'
  Setting an EVAL scope, savestack=37
   4 <<x1>> <[x2]{x3}>    |  1:  IFMATCH[-1]
   3 <<x1> <>[x2]{x3}>    |  3:    OPEN1
   3 <<x1> <>[x2]{x3}>    |  5:    ANYOF[)>\]}]
   4 <<x1>> <[x2]{x3}>    | 16:    CLOSE1
   4 <<x1>> <[x2]{x3}>    | 18:    SUCCEED
                              could match...
   4 <<x1>> <[x2]{x3}>    | 20:  IFMATCH[-0]
   4 <<x1>> <[x2]{x3}>    | 22:    OPEN2
   4 <<x1>> <[x2]{x3}>    | 24:    ANYOF[(<[{]
   5 <<x1>[> <x2]{x3}>    | 35:    CLOSE2
   5 <<x1>[> <x2]{x3}>    | 37:    SUCCEED
                              could match...
   4 <<x1>> <[x2]{x3}>    | 39:  EVAL
  re_eval 0x63a28
Match 2: on left >, on right [
   4 <<x1>> <[x2]{x3}>    | 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]{x3}>    |  1:  IFMATCH[-1]
   4 <<x1>> <[x2]{x3}>    |  3:    OPEN1
   4 <<x1>> <[x2]{x3}>    |  5:    ANYOF[)>\]}]
                              failed...
...
[download]

Using look-behind and capture

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 `(?x)
       (?<=((?-xism:[]>})])))
       ((?-xism:[[<{(]))
       (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}}
+)
   '
size 37 Got 300 bytes for offset annotations.
first at 1
synthetic stclass `ANYOF[(<[{]'.
   1: IFMATCH[-1](20)
   3:   OPEN1(5)
   5:     ANYOF[)>\]}](16)
  16:   CLOSE1(18)
  18:   SUCCEED(0)
  19:   TAIL(20)
  20: OPEN2(22)
  22:   ANYOF[(<[{](33)
  33: CLOSE2(35)
  35: EVAL(37)
  37: END(0)
stclass `ANYOF[(<[{]' minlen 1 with eval 
Offsets: [37]
        17[17] 0[0] 17[1] 0[0] 26[6] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0
+] 0[0] 0[0] 0[0] 33[1] 0[0] 33[0] 33[0] 43[1] 0[0] 52[6] 0[0] 0[0] 0[
+0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 59[1] 0[0] 68[68] 0[0] 136[0] 
 String: <x1>[x2]{x3}(x4)
----------------------------------------
Matching REx `(?x)
       (?<=((?-xism:[]>})])))
       ((?-xism:[[<{(]))
       (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}}
+)
   ...' against `<x1>[x2]{x3}(x4)'
Matching stclass `ANYOF[(<[{]' against `<x1>[x2]{x3}(x4)'
  Setting an EVAL scope, savestack=25
   0 <> <<x1>[x2]{x3}>    |  1:  IFMATCH[-1]
                            failed...
  Setting an EVAL scope, savestack=25
   4 <<x1>> <[x2]{x3}>    |  1:  IFMATCH[-1]
   3 <<x1> <>[x2]{x3}>    |  3:    OPEN1
   3 <<x1> <>[x2]{x3}>    |  5:    ANYOF[)>\]}]
   4 <<x1>> <[x2]{x3}>    | 16:    CLOSE1
   4 <<x1>> <[x2]{x3}>    | 18:    SUCCEED
                              could match...
   4 <<x1>> <[x2]{x3}>    | 20:  OPEN2
   4 <<x1>> <[x2]{x3}>    | 22:  ANYOF[(<[{]
   5 <<x1>[> <x2]{x3}>    | 33:  CLOSE2
   5 <<x1>[> <x2]{x3}>    | 35:  EVAL
  re_eval 0x63a28
Match 1: on left >, on right [
   5 <<x1>[> <x2]{x3}>    | 37:  END
Match successful!
Matching REx `(?x)
       (?<=((?-xism:[]>})])))
       ((?-xism:[[<{(]))
       (?{print qq{Match @{ [++ $count] }: on left $1, on right $2\n}}
+)
   ...' against `x2]{x3}(x4)'
...
[download]

Thank you for your replies and the insights they have given.

Cheers,

JohnGG

In reply to Re^4: Regex code block executes twice per match using look-arounds by johngg
in thread Regex code block executes twice per match using look-arounds by johngg

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