perl regular expressions and empty strings unexpected results

by bdimych (Monk)

on Jan 23, 2012 at 09:39 UTC ( #949344=perlquestion: print w/replies, xml )

Need Help??

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

perl -e 'print ""=~/foo/ and ""=~/bar/'
[download]

prints nothing as expected
but

perl -e 'print ""=~/.*/ and ""=~/bar/'
[download]

prints 1
imho looks strange
I could not find something in documentation
why so?

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Re: perl regular expressions and empty strings unexpected results by moritz (Cardinal) on Jan 23, 2012 at 09:44 UTC
`and` has very loose precedence, so your last example is parsed as `(print "" =~ /.*/) and "" =~ /bar/` [download] The first regex match succeeds (thus the 1), and the result of the second one is discarded. Use `&&` instead of `and` to get the result you want. See also: perlop. Perl 6 - second systems done right	[reply] [d/l] [select]
Re: perl regular expressions and empty strings unexpected results by Anonymous Monk on Jan 23, 2012 at 10:19 UTC
Perl comes with B::Deparse `$ perl -e " print ''=~/./ and ''=~/bar/ " 1 $ perl -MO=Deparse -e " print ''=~/./ and ''=~/bar/ " '' =~ /bar/ if print '' =~ /./; -e syntax OK $ perl -MO=Deparse,-p -e " print ''=~/./ and ''=~/bar/ " (print(('' =~ /.*/)) and ('' =~ /bar/)); -e syntax OK` [download]	[reply] [d/l]

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