Perl, Using eval in regex

Using eval in regex

Author	Message
WizardsOfTheLowCos #1 / 6	Using eval in regex I'm trying to write a regex matching substrings with reversed substrings : xxxabcdexxxedcbaxxx _____ _____ Do you know if I can use eval in a regular expression for that ? if ( /(\w+)\w{1,10}(eval(reverse \1))/g ) { print "match $1 \n"; Quote: } else { print " no match \n"; Quote: }; It does not seem to work. Any ideas ?
Sat, 29 Jan 2005 19:05:26 GMT

Steven Ku #2 / 6	Using eval in regex Quote: > I'm trying to write a regex matching substrings with reversed > substrings : > xxxabcdexxxedcbaxxx > _____ _____ > Do you know if I can use eval in a regular expression for that ? > if ( /(\w+)\w{1,10}(eval(reverse \1))/g ) { > print "match $1 \n"; > } > else { > print " no match \n"; > }; > It does not seem to work. Any ideas ? You could try the "highly experimental" extended regex feature (as explained in perlre): while (<DATA>) { if (/(\w+)(?:\w*?)(??{scalar reverse $1})/) { print "$1\n"; } Quote: } __DATA__ This_is_in_perlre_si_sihT in_perlre_try_perldoc_erlrep_ni xxxabcdexxxedcbaxxx -- Cheers, Steven
Sat, 29 Jan 2005 20:11:03 GMT

WizardsOfTheLowCos #3 / 6	Using eval in regex Thanks a lot Steven, it worked great. By the way, why does this regex requires $1 and not /1 ? I thought that $1 works only outside of regex. On Tue, 13 Aug 2002 12:11:03 -0700, Steven Kuo Quote: >> I'm trying to write a regex matching substrings with reversed >> substrings : >> xxxabcdexxxedcbaxxx >> _____ _____ >> Do you know if I can use eval in a regular expression for that ? >> if ( /(\w+)\w{1,10}(eval(reverse \1))/g ) { >> print "match $1 \n"; >> } >> else { >> print " no match \n"; >> }; >> It does not seem to work. Any ideas ? >You could try the "highly experimental" extended regex feature (as >explained in perlre): >while (<DATA>) { > if (/(\w+)(?:\w*?)(??{scalar reverse $1})/) { > print "$1\n"; > } >} >__DATA__ >This_is_in_perlre_si_sihT >in_perlre_try_perldoc_erlrep_ni >xxxabcdexxxedcbaxxx
Sat, 29 Jan 2005 21:21:40 GMT

Steven Ku #4 / 6	Using eval in regex Quote: > On Tue, 13 Aug 2002 12:11:03 -0700, Steven Kuo > >> I'm trying to write a regex matching substrings with reversed > >> substrings : > >> ... > >> It does not seem to work. Any ideas ? > >You could try the "highly experimental" extended regex feature (as > >explained in perlre): > >while (<DATA>) { > > if (/(\w+)(?:\w*?)(??{scalar reverse $1})/) { > > print "$1\n"; > > } > >} > > ... > Thanks a lot Steven, it worked great. > By the way, why does this regex requires $1 and not /1 ? > I thought that $1 works only outside of regex. $1 is determined at run time. The code which produces the regular expression is also evaluated at run time. That's why we use $1 instead of \1. The experts in c.l.p.m. can expand upon that explanation (for my benefit as well)... -- Cheers, Steven
Sat, 29 Jan 2005 22:23:49 GMT

Steffen Muelle #5 / 6	Using eval in regex Quote: [...] > > Thanks a lot Steven, it worked great. > > By the way, why does this regex requires $1 and not /1 ? > > I thought that $1 works only outside of regex. > $1 is determined at run time. The code which produces the regular > expression is also evaluated at run time. That's why we use $1 > instead of \1. > The experts in c.l.p.m. can expand upon that explanation (for my > benefit as well)... Hrm, well, expert, no. By saying that you'll scare off everybody who doesn't consider himself an expert. Thanks for the compliment anyway. ;) Anyway, I generally advise $1 over \1 because... perldoc perlre --- Warning on \1 vs $1 Some people get too used to writing things like: $pattern =~ s/(\W)/\\\1/g; This is grandfathered for the RHS of a substitute to avoid shocking the sed {filter}s, but it's a dirty habit to get into. That's because in PerlThink, the righthand side of a s/// is a double-quoted string. \1 in the usual double-quoted string means a control-A. The customary Unix meaning of \1 is kludged in for s///. However, if you get into the habit of doing that, you get yourself into trouble if you then add an /e modifier. s/(\d+)/ \1 + 1 /eg; # causes warning under -w Or if you try to do s/(\d+)/\1000/; You can't disambiguate that by saying \{1}000, whereas you can fix it with ${1}000. The operation of interpolation should not be confused with the operation of matching a backreference. Certainly they mean two different things on the left side of the s///. --- Does that clear things up? Steffen
Sat, 29 Jan 2005 23:42:29 GMT

Bart Lateu #6 / 6	Using eval in regex Quote: >By the way, why does this regex requires $1 and not /1 ? >I thought that $1 works only outside of regex. Yes, and no. \1 is to be used only in plain sections in the regex. But (??{...}) is special, as what is inside it is not a plain regex section, but code (thus requiring $1, \1 will not work there) which evaluates to something that gets compiled into a regex. So if this code produced something contain a backslash followed by a "1", it normally would work there (exception: when the backslash itself got escaped). BTW I don't expect that this will do what you want. You don't set the minimum requirement for the length of the delimiter and its reverse higher than one. So if you attempt to apply your thing on "xxxabcdexxxxedcbaxxx", it would match an "x" followed by an "x". -- Bart.
Tue, 01 Feb 2005 14:34:40 GMT

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