What does $1 mean in Perl?
RegexPerlRegex Problem Overview
What does $1 mean in Perl? Further, what does $2 mean? How many $number variables are there?
Regex Solutions
Solution 1 - Regex
The $number variables contain the parts of the string that matched the capture groups ( ... ) in the pattern for your last regex match if the match was successful.
For example, take the following string:
$text = "the quick brown fox jumps over the lazy dog.";
After the statement
$text =~ m/ (b.+?) /;
$1 equals the text "brown".
Solution 2 - Regex
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.
Solution 3 - Regex
$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";
Solution 4 - Regex
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
Solution 5 - Regex
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.)
Solution 6 - Regex
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';
Solution 7 - Regex
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.
Solution 8 - Regex
I would suspect that there can be as many as 2**32 -1 numbered match variables, on a 32-bit compiled Perl binary.