http://www.perlmonks.org?node_id=570826

greatdane has asked for the wisdom of the Perl Monks concerning the following question:

I must be doing something stupid...

Why would the following while loop hang if the search string is not found in $PS ? Thanks in advance! 

while ($PS =~ m/\G.*?Startpoint:\s*(\S+).*?Endpoint:\s*(\S+).*?Path Gr
+oup:\s*(\S+).*?slack\s*\(.*?\)\s*(\S+)/gcsm) { <whatever> }

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Its something to do with the depth of my search. It only hangs because all of the way through "slack" is found but "slack" is not found. If I make "Path Group" not found, then it does not hang. Also, if I make "slack" found, then it does not hang.

Update: its not the while that is hanging. Its actually the search that is hanging. The program hangs even when I take the search out of the while expression.

Code tags added by GrandFather

Replies are listed 'Best First'.
Re: search hang trouble
by greatdane (Beadle) on Sep 01, 2006 at 22:56 UTC
    I am now believing that the search is not hung but massively busy because of the cascaded non-greedy search pattern. The search hits on the first 3 specific strings but fails on the 4th specific string parsing a file with 50 cases of these failures. This results in a massive amount of nearly passing patterns.

    To solve my "seemingly hung" problem, I added a pre-check to the search string to see that the search is likely needed.

    Thanks for your help. This case can be considered closed.

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Re: search hang trouble
by GrandFather (Saint) on Sep 01, 2006 at 21:10 UTC

    How about giving us a sample string that causes the match to "hang"?

    DWIM is Perl's answer to Gödel
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Re: search hang trouble
by mreece (Friar) on Sep 01, 2006 at 21:21 UTC
    what does the /c do in /gcsm?
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[d/l]
[select]
      search options: 
      c   Do not reset search pos on a failed match when /g is in effect.
g   Match globally, i.e., find all occurrences.
i   Do case-insensitive pattern matching.
m   Treat string as multiple lines.
o   Compile pattern only once.
s   Treat string as single line.
x   Use extended regular expressions.

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[d/l]
      ah, i found it buried in perldoc perlretut:
      If you don't want the position reset after failure to match, add the "//c", as in "/regexp/gc".
      odd that it does not appear to be mentioned in perldoc perlre..
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        Because it's not a regex flag, per se. It's a pattern matching operator flag. perlre only covers /i, /m, /s, and /x.
        Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
        How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart
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