These are called "match variables". As previously mentioned they contain the text from your last regular expression match.
More information is in Essential Perl. (Ctrl + F for 'Match Variables' to find the corresponding section.)
In general, questions regarding "magic" variables in Perl can be answered by looking in the Perl predefined variables documentation a la:
perldoc perlvar
However, when you search this documentation for $1, etc., you'll find references in a number of places except the section on these "digit" variables. You have to search for
$<digits>
I would have added this to Brian's answer either by commenting or editing, but I don't have enough rep. If someone adds this I'll remove this answer.
Since you asked about the capture groups, you might want to know about $+ too... Pretty useful...
use Data::Dumper;
$text = "hiabc ihabc ads byexx eybxx";
while ($text =~ /(hi|ih)abc|(bye|eyb)xx/igs)
{
print Dumper $+;
}
OUTPUT:
$VAR1 = 'hi';
$VAR1 = 'ih';
$VAR1 = 'bye';
$VAR1 = 'eyb';
As others have pointed out, the $x are capture variables for regular expressions, allowing you to reference sections of a matched pattern.
Perl also supports named captures which might be easier for humans to remember in some cases.
Given input: 111 222
/(\d+)\s+(\d+)/
$1 is 111
$2 is 222
One could also say:
/(?<myvara>\d+)\s+(?<myvarb>\d+)/
$+{myvara} is 111
$+{myvarb} is 222
The number variables are the matches from the last successful match or substitution operator you applied:
my $string = 'abcdefghi';
if ($string =~ /(abc)def(ghi)/) {
print "I found $1 and $2\n";
}
Always test that the match or substitution was successful before using $1
and so on. Otherwise, you might pick up the leftovers from another operation.
Perl regular expressions are documented in perlre.
I would suspect that there can be as many as 2**32 -1
numbered match variables, on a 32-bit compiled Perl binary.
The variables $1 .. $9 are also read only variables so you can't implicitly assign a value to them:
$1 = 'foo'; print $1;
That will return an error: Modification of a read-only value attempted at script line 1.
You also can't use numbers for the beginning of variable names:
$1foo = 'foo'; print $1foo;
The above will also return an error.
$1, $2, etc will contain the value of captures from the last successful match - it's important to check whether the match succeeded before accessing them, i.e.
if ( $var =~ m/( )/ ) { # use $1 etc... }
An example of the problem - $1 contains 'Quick' in both print statements below:
#!/usr/bin/perl
'Quick brown fox' =~ m{ ( quick ) }ix;
print "Found: $1\n";
'Lazy dog' =~ m{ ( quick ) }ix;
print "Found: $1\n";
Source: Stackoverflow.com