How can I extract the nth match in a Perl regex?

Question

Is it possible to extract a match n ^th in a string of one-word words?

use strict; use warnings; my $string1 = "'I want to' 'extract the word' 'Perl','from this string'"; my $string2 = "'What about','getting','Perl','from','here','?'"; sub extract_quoted { my ($string, $index) = @_; my ($wanted) = $string =~ /some_regex_using _$index/; return $wanted; } extract_wanted ($string1, 3); # Should return 'Perl', with quotes extract_wanted ($string2, 3); # Should return 'Perl', with quotes 

Answer 1

You may try:

 sub extract_quoted { my ($string, $index) = @_; while($string =~ /'(.*?)'/g) { $index--; return $1 if(! $index); # return $1 if index became 0. } return; # not found - returns undef or () depending on context. } 

Answer 2

See this question: How to save consistent parts of a regular expression in Perl? and this answer (switch / g is a trick)

Answer 3

The match-g operator evaluated in the context of the array gives an array of matches. Therefore:

 @matches = $string =~ /'(.*?)'/g; $matches[$index-1]; 

- This is one way to get what you want.

Answer 4

This should work:

 sub extract_quoted { my ($string, $index) = @_; my $wanted = ($string =~ /'(.*?)'/g)[$index]; return $wanted; } 

Answer 5

 use strict; use warnings; use Text::ParseWords; my $string1 = q{'I want to' 'extract the word' 'Perl','from this string'}; my $string2 = q{'What about', 'getting','Perl','from','here','?'}; print extract_wanted($_, 3), "\n" for ($string1, $string2); sub extract_wanted { my ($string, $index) = @_; my $wanted = (parse_line '(:?\s|,)+', 0, $string)[$index - 1]; return unless defined $wanted; return $wanted; } 

Conclusion:

  Perl
 Perl 

Answer 6

It may look ugly, but

 my $quoted = qr/'[^']+'/; # ' fix Stackoverflow highlighting my %_extract_wanted_cache; sub extract_wanted_memo { my($string, $index) = @_; $string =~ ($_extract_wanted_cache{$index} ||= qr/^(?:.*?$quoted.*?){@{[$index-1]}}($quoted)/) ? $1 : (); } 

Benchmarking suggests that this may be useful:

 sub extract_wanted { my($string, $index) = @_; $string =~ /^(?:.*?$quoted.*?){@{[$index-1]}}($quoted)/ ? $1 : (); } sub extract_wanted_gindex { my($string, $index) = @_; ($string =~ /$quoted/g)[$index-1]; } use Benchmark; timethese -1 => { nocache => sub { extract_wanted $string2, 3 }, memoize => sub { extract_wanted_memo $string2, 3 }, index => sub { extract_wanted_gindex $string2, 3 }, nocache_fail => sub { extract_wanted $string2, 100 }, memoize_fail => sub { extract_wanted_memo $string2, 100 }, index_fail => sub { extract_wanted_gindex $string2, 100 }, } 

Results:

  Benchmark: 
 running
  index, index_fail, memoize, memoize_fail, nocache, nocache_fail
  for at least 1 CPU seconds
 ...

      index: 1 w / c secs (1.04 usr + 0.00 sys = 1.04 CPU) @ 183794.23 / s (n = 191146)
 index_fail: 1 w / c secs (1.03 usr + 0.00 sys = 1.03 CPU) @ 185578.64 / s (n = 191146)
    memoize: 1 w / c secs (1.00 usr + 0.00 sys = 1.00 CPU) @ 264664.00 / s (n = 264664)
 memoize_fail: 0 w / c secs (1.03 usr + 0.00 sys = 1.03 CPU) @ 835106.80 / s (n = 860160)
    nocache: 0 w / c secs (1.03 usr + 0.00 sys = 1.03 CPU) @ 196495.15 / s (n = 202390)
 nocache_fail: 2 w / c secs (1.03 usr + 0.00 sys = 1.03 CPU) @ 445390.29 / s (n = 458752) 

Answer 7

 sub extract_quoted { my ($string,$index) = @_; $c=1; @s = split /\'/,$string; for($i=1;$i<=$#s;$i+=2){ $c==$index && return $s[$i]; $c++; } }