$ perl -Mre=debug -e '$_ = q!(b).!; /\((?=a|b|c)\)/ && do { print qq{m
+atch! $&\n}; };'
Compiling REx "\((?=a|b|c)\)"
Final program:
   1: EXACT <(> (3)
   3: IFMATCH[0] (16)
   5:   TRIE-EXACT[abc] (14)
        <a>
        <b>
        <c>
  14:   SUCCEED (0)
  15: TAIL (16)
  16: EXACT <)> (18)
  18: END (0)
anchored "()" at 0 (checking anchored) minlen 2
Guessing start of match in sv for REx "\((?=a|b|c)\)" against "(b)."
Did not find anchored substr "()"...
Match rejected by optimizer
Freeing REx: "\((?=a|b|c)\)"





$ perl -Mre=debug -e '$_ = q!(b).!; /\((?=a|b|c)/ && do { print qq{mat
+ch! $&\n}; };'
Compiling REx "\((?=a|b|c)"
Final program:
   1: EXACT <(> (3)
   3: IFMATCH[0] (16)
   5:   TRIE-EXACT[abc] (14)
        <a>
        <b>
        <c>
  14:   SUCCEED (0)
  15: TAIL (16)
  16: END (0)
anchored "(" at 0 (checking anchored) minlen 1
Guessing start of match in sv for REx "\((?=a|b|c)" against "(b)."
Found anchored substr "(" at offset 0...
Guessed: match at offset 0
Matching REx "\((?=a|b|c)" against "(b)."
   0 <> <(b).>               |  1:EXACT <(>(3)
   1 <(> <b).>               |  3:IFMATCH[0](16)
   1 <(> <b).>               |  5:  TRIE-EXACT[abc](14)
   1 <(> <b).>               |      State:    1 Accepted:    0 Charid:
+  2 CP:  62 After State:    3
   2 <(b> <).>               |      State:    3 Accepted:    1 Charid:
+  2 CP:   0 After State:    0
                                    got 1 possible matches
                                    only one match left: #2 <b>
   2 <(b> <).>               | 14:  SUCCEED(0)
                                    subpattern success...
   1 <(> <b).>               | 16:END(0)
Match successful!
match! (
Freeing REx: "\((?=a|b|c)"
[download]