•Re^3: You *Can* Catch errors in closing lexical filehandles

The regular expression captured submatches ($1, $2, ...) are now more consistently unset if the match fails, instead of leaving false data lying around in them.

I believe this is referring to the fact that if a match has $1..$6 in one successful match, then only $1..$2 in the next match, that $3..$6 are more carefully set to undef now. I remember there being a patch to that effect.

If they were to change the other behavior, it would no longer be (usefully) upward compatible. This would have been a grave mistake, caught by someone (like me).

The reason the variables are not unset on failure is to permit:

if ($string = /... ( ... ) .../) { # overall match
  if ($1 =~ /foo/) { # and $1 contains foo
  ...
  } elsif ($1 =~ /bar/) { # nope?  how about bar
  ...
  }
}
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If $1 reset on the failed match on the second line, then we couldn't do the third match. This has been by design in Perl for over a decade now. (And I just verified that 5.8.5 doesn't unset the variables, so it works as it always has.)

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

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Re^4: You Can Catch errors in closing lexical filehandles by ihb (Deacon) on Sep 27, 2004 at 23:09 UTC
And I just verified that 5.8.5 doesn't unset the variables, so it works as it always has. Ah, indeed. I forgot that I've also tried it with the same result as you. I brought it up on freenode #perl. I considered it a bug and I still do. It's either a documentation bug or a perl bug. I don't think the documentation can be interpreted as you suggest. It clearly says "if the match fails". I believe this is referring to the fact that if a match has $1..$6 in one successful match, then only $1..$2 in the next match, that $3..$6 are more carefully set to undef now. That's the behaviour I get for perl version 5.005_03 built for sun4-solaris. I interpret the "more consistently" part as a mismatch unsets all, since that behaviour would be more consistant with how it already works when `$3` .. `$6` is unset/not matched; failing a match would mean "you didn't match `$1` .. `$n` so it should be unset". Whether or not the behaviour is more consistant I won't argue over, but this is how I interpret the documentation. I assumed this behaviour change was due to some considering that the values in the regex variables after a failed match are "false data", as the documentation calls it. I assumed it was an attempt to make Perl DWIM, for some definition of I. `ihb` Read argumentation in its context!	[reply] [d/l] [select]

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Re^4: You *Can* Catch errors in closing lexical filehandles
by ihb (Deacon) on Sep 27, 2004 at 23:09 UTC

And I just verified that 5.8.5 doesn't unset the variables, so it works as it always has.

Ah, indeed. I forgot that I've also tried it with the same result as you. I brought it up on freenode #perl. I considered it a bug and I still do. It's either a documentation bug or a perl bug. I don't think the documentation can be interpreted as you suggest. It clearly says "if the match fails".

I believe this is referring to the fact that if a match has $1..$6 in one successful match, then only $1..$2 in the next match, that $3..$6 are more carefully set to undef now.

That's the behaviour I get for perl version 5.005_03 built for sun4-solaris. I interpret the "more consistently" part as a mismatch unsets all, since that behaviour would be more consistant with how it already works when $3 .. $6 is unset/not matched; failing a match would mean "you didn't match $1 .. $n so it should be unset". Whether or not the behaviour is more consistant I won't argue over, but this is how I interpret the documentation.

I assumed this behaviour change was due to some considering that the values in the regex variables after a failed match are "false data", as the documentation calls it. I assumed it was an attempt to make Perl DWIM, for some definition of I.

ihb

Read argumentation in its context!

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