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!
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!
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
+ ("......'
