Re^3: Use of uninitialized value $1

"These variables keep their value until a new regex is successfully matched or dynamic scope is exited."

I'm not sure that is true. I remember a node that fairly recently talked about this, but I couldn't find it, only What's happening to my $1?.

What I remember is that a failed match could in some circumstances modify $1. So $1 is only reliable if the match succeeds (can't rely upon it being what it was after the previous regex after a failed match). I also remember that this behavior is Perl version specific.

I haven't run across this in my code as I do whatever I'm going to do with $1 right after the regex and have never written code that decides whether $1 changed or not. I only use $1 if the match succeeded.

Yes, this is a nit, but Perl Monks is good about resolving these nits and details.

Comment on Re^3: Use of uninitialized value $1

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Re^4: Use of uninitialized value $1 by AnomalousMonk (Archbishop) on Aug 26, 2016 at 01:06 UTC
Yes, this is a nit ... Regular expressions, love them though I do, consist in a vast collection of nits IMHO. I'm not sure that [these variables keep their value] is true. I remember a node that fairly recently talked about this, but I couldn't find it ... What I remember is that a failed match could in some circumstances modify $1. Now that you mention it, I seem to remember something along those lines myself. Unfortunately, my Super Search-fu is no better than yours, and I could come up with no pertinent node. Have to keep searching, searching... (Maybe it had to do with the match portion of `s///` having the same effects on regex variables as `m//` does?)	[reply] [d/l] [select]
Re^5: Use of uninitialized value $1 by Marshall (Canon) on Aug 26, 2016 at 03:15 UTC
perlre says: NOTE: Failed matches in Perl do not reset the match variables, which makes it easier to write code that tests for a series of more specific cases and remembers the best match. However, we both remember something different about that. I hope some other Monk can point us in the right direction. Update: After some reflection, my memory recalls some situation where there were several capture groups, $1,$2,$3. Overall, the match failed, but the first part ($1) did match and change $1 although the rest of the expression $2,$3 didn't match and didn't change. This behavior was version specific. So, this is a cautionary note that despite what the doc says, there may be some "bite you" case out there that my super-search foo is not able to find.	[reply]