Those greedy quantifiers!

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

Hello!

Trying to generate a list of all extensions in a directory, sans filenames. So, it seems

/\.(.*)?$/;
[download]

should work, but it matches

RF2A7FF2E.TMP
 PROFILE.DOC
BETTER..JPG
-.JPG
[download]

So, I tried moving the quantifier inside the parents:

/\.(.*?)$/;
[download]

to the same results. No one ever promised regexes would be easy... What did I miss?

Cheers! ~P

Ready.
poke 53280,0
poke 53281,0
ctrl+2 load "zork",8,1

West of House

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Re: Those greedy quantifiers! by jwkrahn (Abbot) on Apr 01, 2010 at 01:36 UTC
What did I miss? The `\.` at the beginning of the pattern will match the first period that satisfies the pattern, it doesn't matter if the `.` is greedy or not. You probably want `/\.([^.])\z/` instead.	[reply] [d/l] [select]
Re: Those greedy quantifiers! by ikegami (Patriarch) on Apr 01, 2010 at 01:46 UTC
`?` is a quantifier in your first snippet (quantified atom matches optionally matches once) because it's not preceded by another quantifier. (And it happens to be useless in your pattern.) `?` is a greediness modifier (makes it non-greedy) in your second snippet because it's preceded by a quantifier. There's no reason to alter greediness here. Solution: `my %seen; my @exts = grep !$seen{$_}++, map /\.([^.]+)\z/, @file_names;` [download]	[reply] [d/l] [select]
Re: Those greedy quantifiers! by Anonymous Monk on Apr 01, 2010 at 01:13 UTC
use YAPE::Regex::Explain; print YAPE::Regex::Explain->new( '\.([^.]+)$' )->explain; __END__ The regular expression: (?-imsx:\.([^.]+)$) 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): ---------------------------------------------------------------------- \. '.' ---------------------------------------------------------------------- ( group and capture to \1: ---------------------------------------------------------------------- [^.]+ any character except: '.' (1 or more times (matching the most amount possible)) ---------------------------------------------------------------------- ) end of \1 ---------------------------------------------------------------------- $ before an optional \n, and the end of the string ---------------------------------------------------------------------- ) end of grouping ---------------------------------------------------------------------- [download]	[reply] [d/l]