> they match the empty string - and within that empty string, the first capture is undefined.

to elaborate further here the proof that it's matching, just at the wrong position

  DB<1> $foo = 'a=b';

  DB<2> $foo = 'x' . $foo

  DB<3> p scalar $foo =~/(?:a=([a-z]+))?/;
1
  DB<4> p $1

  DB<5>

  DB<6> p scalar $foo =~/(?:a=([a-z]+))?/g;
1
  DB<7> p pos $foo
0
  DB<8>

[download]

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

In reply to Re^2: Making a subpattern optional with ? causes subpattern match to fail. I am confused as to why. by LanX
in thread Making a subpattern optional with ? causes subpattern match to fail. I am confused as to why. by msouth

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