http://qs1969.pair.com?node_id=199438

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

Need help figuring this out: 
.*${name}-*

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It is saying any letters and then a literal and then ...?
I am curious to know what this regular expression is doing? Am I on the right track??

Replies are listed 'Best First'.
Re: Regular Expression mystery
by PodMaster (Abbot) on Sep 20, 2002 at 11:15 UTC
    Don't ever guess again. 
    use YAPE::Regex::Explain;
die YAPE::Regex::Explain->new(qr/.*${name}-*/)->explain;
__END__
The regular expression:

(?-imsx:.*-*)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  -*                       '-' (0 or more times (matching the most
                           amount possible))
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

    [download]
    update:
    In case you were wondering where the (?-imsx:.*-*) comes from, well perl -e"print qr/asdf/", perl -e"print ref qr/a/" and since ${name} is not defined anywhere in the above snippet, that part don't show up in the explanation. Here's an alternative 
    use YAPE::Regex::Explain;
my $name = 'asdf';
die YAPE::Regex::Explain->new(qr/.*${name}-*/)->explain;
__END__
The regular expression:

(?-imsx:.*asdf-*)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  asdf                     'asdf'
----------------------------------------------------------------------
  -*                       '-' (0 or more times (matching the most
                           amount possible))
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

    [download]

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    ** The Third rule of perl club is a statement of fact: pod is sexy.

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[d/l]
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      I got alot of answers here. Thanks so much to all of you!
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Re: Regular Expression mystery
by davorg (Chancellor) on Sep 20, 2002 at 11:13 UTC
    • .* - zero or more of any character
    • ${name} - the contents of $name
    • -* - zero or more '-' characters.
    --
    <http://www.dave.org.uk>

    "The first rule of Perl club is you do not talk about Perl club."
    -- Chip Salzenberg

[reply]
Re: Regular Expression mystery
by thinker (Parson) on Sep 20, 2002 at 11:18 UTC
    Hi

    Imagine the variable $name contained "foo", the regex will expand to .*foo-* , which will be true if the line contains any character, followed somewhwere with foo, and one or more - signs.

    cheers

    thinker

    update As can be seen in the replies from the less careless monks who posted before me, the - sign need not appear. apologies.
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