Backslashed "@" in reg exes

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

The following line in LWP::RobotUA caught my eye:

...unless $cnf{from} && $cnf{from} =~ m/\@/;
[download]

Can anyone tell me why the "@" is escaped? The code prompted me to run this little test:

my $string = 'one@two.com';
if( $string =~ m/\@/ ) {
  print 'yes';
}
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The above code prints 'yes' with and without the backslash. So what exactly is that backslash doing?

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Comment on Backslashed "@" in reg exes Select or Download Code

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Re: Backslashed "@" in reg exes by Tanktalus (Canon) on Nov 13, 2005 at 03:36 UTC
LWP::RobotUA may not need it there. But for longer strings, it's just a good habit to be in. For example, `if ($string =~ m/@a/) {` [download] you'll get `Possible unintended interpolation of @a in string at ./y.pl line 7.` [download] as a warning message under `use warnings;` (which you are doing, right?). But with the \, you'll get exactly what you wanted - an @-sigil followed by the letter a.	[reply] [d/l] [select]
Re: Backslashed "@" in reg exes by Zaxo (Archbishop) on Nov 13, 2005 at 04:36 UTC
A stringy regex appearing in a match is treated as between double quotes. That makes perl attempt to interpolate both "@" and "$" followed by legal variable names. Interpolation is done before the regex is compiled. Since an array being interpolated is joined by the value of the $" variable, I like to use a local value of $" to construct big alternations: `my $alt_foo_re = do { local $" = '\|'; qr/@foo/; }; # same as: # my $alt_foo_re = do { # my $alt_foo = join '\|', @foo; # qr/$alt_foo/; # }` [download] After Compline, Zaxo	[reply] [d/l]
Re: Backslashed "@" in reg exes by pg (Canon) on Nov 13, 2005 at 03:59 UTC
The backslash is not needed in this particular case, but it could be a good habit to have it there. Personally, I would have the backslash in front of the @, any time when interpolation is not desired, which seems a little bit more consistent to me.	[reply]