Matching @ in string

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

Dear all,

I must be missing something obvious, but I don't understand why

perl -e 'if ("h@" =~ /(\@)/ ) { print $1 }'

prints the at-sign, but

perl -e 'if ("h@t" =~ /(\@)/ ) { print $1 }'

doesn't.

Thanks,

loris

"It took Loris ten minutes to eat a satsuma . . . twenty minutes to get from one end of his branch to the other . . . and an hour to scratch his bottom. But Slow Loris didn't care. He had a secret . . ." (from "Slow Loris" by Alexis Deacon)

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Re: Matching @ in string by lima1 (Curate) on Nov 22, 2007 at 13:22 UTC
You have introduced the array @t which is interpolated. So you have to quote the @ (or write q{h@t}). `perl -e 'if ("h\@t" =~ /(\@)/ ) { print $1 }'` [download] Example for an array interpolation: `#!/usr/bin/perl use strict; use warnings; my @friends = ('Margaret', 'Richard', 'Carolyn', 'Rohan', 'Cathy', 'Yukiko'); print "Friends: @friends\n";` [download]	[reply] [d/l] [select]
Re^2: Matching @ in string by loris (Hermit) on Nov 22, 2007 at 13:49 UTC
OK, so I was missing something obvious. But if I don't want to modify the string by escaping the @ and I want to run program as a one-liner, how do I do it if I can't escape the single quotes as you point out here? Thanks, loris "It took Loris ten minutes to eat a satsuma . . . twenty minutes to get from one end of his branch to the other . . . and an hour to scratch his bottom. But Slow Loris didn't care. He had a secret . . ." (from "Slow Loris" by Alexis Deacon)	[reply]
Re^3: Matching @ in string by dwu (Monk) on Nov 22, 2007 at 13:52 UTC
`perl -e 'if (q{h@t} =~ /(\@)/ ) { print $1 }'` [download] See perlop for the `q` operator; look under "Regexp Quote-Like Operators", and a few scrolls down from that :) PS: Your link to Re^3: Matching @ in string is broken because you did `node=id` instead of `node_id` - see What shortcuts can I use for linking to other information? for better, more efficient means of linking to information on Perlmonks.	[reply] [d/l] [select]
Re^3: Matching @ in string by mwah (Hermit) on Nov 22, 2007 at 14:34 UTC
... don't want to modify the string by escaping the @ and I want to run program as a one-liner, how do I do it if I can't escape the single quotes Try: ... `perl -e "print 'h@t'=~ /\@/g"` [download] or even `echo 'h@t' \| perl -ne 'print /\@/g'` [download] ... depending on the type of problem you are about to solve. Regards mwa	[reply] [d/l] [select]
Re: Matching @ in string by dwu (Monk) on Nov 22, 2007 at 13:24 UTC
I'm suspecting because perl interprets `@t` within the string `h@t` as the array `@t`, since double quotes (`""`) will cause variables to be interpolated. Inserting both tests into a short script: `#!/usr/bin/perl use strict; use warnings; if ("h@" =~ /(\@)/ ) { print $1 } if ("h@t" =~ /(\@)/ ) { print $1 }` [download] produces: `Possible unintended interpolation of @t in string at ./scratch.pl line + 6. Global symbol "@t" requires explicit package name at ./scratch.pl line + 6. Execution of ./scratch.pl aborted due to compilation errors.` [download] Doing: `#!/usr/bin/perl use strict; use warnings; if ('h@' =~ /(\@)/ ) { print $1 } if ('h@t' =~ /(\@)/ ) { print $1 }` [download] produces: `@@` As expected. HTH.	[reply] [d/l] [select]
Re: Matching @ in string by tinita (Parson) on Nov 22, 2007 at 15:40 UTC
please always use warnings in perl oneliners. i've never understood why people use warnings in their scripts and leave it out in oneliners. if i write a oneliner i automatically start it with `-wle`. this has probably saved me lots of time. `Possible unintended interpolation of @t in string at -e line 1. Name "main::t" used only once: possible typo at -e line 1.` [download]	[reply] [d/l] [select]
Re^2: Matching @ in string by loris (Hermit) on Nov 23, 2007 at 07:49 UTC
I am sure this is very good advice, but in the present case `perl -elw 'if ("h@t" =~ /(\@)/ ) { print $1 }'` doesn't produce any warning (I'm using 5.8.8 under Cygwin and Debian). How did you get the message about the unintended interpolation? loris "It took Loris ten minutes to eat a satsuma . . . twenty minutes to get from one end of his branch to the other . . . and an hour to scratch his bottom. But Slow Loris didn't care. He had a secret . . ." (from "Slow Loris" by Alexis Deacon)	[reply] [d/l] [select]
Re^3: Matching @ in string by tinita (Parson) on Nov 23, 2007 at 08:08 UTC
`perl -elw ...` i said -wle not -elw =) -elw doesn't produce any output.	[reply] [d/l]
Re: Matching @ in string by erroneousBollock (Curate) on Nov 22, 2007 at 13:24 UTC
`perl -e 'if ("h@t" =~ /(\@)/ ) { print $1 }'` "h@t" looks to Perl like you're trying to interpolate the array `@t` (the previous case doesn't because the quote ends the string early. `perl -e 'if ("h\@t" =~ /(\@)/ ) { print $1 }'` or ~~`perl -e 'if (\'h@t\' =~ /(\@)/ ) { print $1 }'`~~ should work fine. Update: err, ooops :-) -David	[reply] [d/l] [select]
Re^2: Matching @ in string by dwu (Monk) on Nov 22, 2007 at 13:28 UTC
cough $ perl -e 'if (\'h@t\' =~ /(\@)/ ) { print $1 }' -bash: syntax error near unexpected token `(' [download] Not sure why - does escaping the single quote not work?	[reply] [d/l]
Re^3: Matching @ in string by lima1 (Curate) on Nov 22, 2007 at 13:34 UTC
Because the bash does not change anything in the -e ' .. ' string. And `if (\'h@t\' =~ /(\@)/ ) {` [download] is no valid perl. And the -e string ends after `if (\`.	[reply] [d/l]

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