Do you know what kinds of things can go wrong?
Backtracking can screw things up:
my $count;
'ac' =~
/
a (?{ $count++ }) b
|
a (?{ $count++ }) c
/x;
# 1. Matches 'a' in first branch.
# 2. Increments $count to 1.
# 3. Fails to match 'b'.
# 4. Matches 'a' in second branch.
# 5. Increments $count to 2.
# 6. Matches 'c'.
print("$count\n"); # 2
The fix is to use local. When the regexp backtracks through a local, the old value is restored. The old value is also restored when the regexp succesfully matches, so you need to save the result.
my $count;
our $c = 0;
'ac' =~
/
(?:
a (?{ local $c = $c + 1 }) b
|
a (?{ local $c = $c + 1 }) c
)
(?{ $count = $c }) # Save result.
/x;
# 1. Matches 'a' in first branch.
# 2. Increments $c to 1.
# 3. Fails to match 'b'.
# 4. Undoes increment ($c = 0).
# 5. Matches 'a' in second branch.
# 6. Increments $c to 1.
# 7. Matches 'c'.
# 8. $count = $c.
print("$count\n"); # 1
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