How does pattern matching work in Perl?

问题

I want to know how pattern matching works in Perl.

My code is:

my $var = "VP KDC T. 20, pgcet. 5, Ch. 415, Refs %50 Annos";

if($var =~ m/(.*)\,(.*)/sgi)
{
    print "$1\n$2";
}

I learnt that the first occurrence of comma should be matched. but here the last occurrence is being matched. The output I got is:

VP KDC T. 20, pgcet. 5, Ch. 415
 Refs %50 Annos

Can someone please explain me how this matching works?

回答1:

From docs:

By default, a quantified subpattern is "greedy", that is, it will match as many times as possible (given a particular starting location) while still allowing the rest of the pattern to match

So, first (.*) will take as much as possible.

Simple workaround is using non-greedy quantifier: *?. Or match not every character, but all except comma: ([^,]*).

回答2:

Greedy and Ungreedy Matching

Perl regular expressions normally match the longest string possible.

For instance:

my($text) = "mississippi";
$text =~ m/(i.*s)/;
print $1 . "\n";

Run the preceding code, and here's what you get:

ississ

It matches the first i, the last s, and everything in between them. But what if you want to match the first i to the s most closely following it? Use this code:

my($text) = "mississippi";
$text =~ m/(i.*?s)/;
print $1 . "\n";

Now look what the code produces:

is

Clearly, the use of the question mark makes the match ungreedy. But theres another problem in that regular expressions always try to match as early as possible.

Source: http://www.troubleshooters.com/codecorn/littperl/perlreg.htm

回答3:

Use question mark in your regex:

if($var =~ m/(.*?)\,(.*)/sgi)
{
    print "$1\n$2";
}

So:

(.*)\, means: "match as much characters as you can as long as there will be a comma after them"
(.*?)\, means: "match any characters until you stumble upon a comma"

回答4:

(.*)\, -you might expect that it will match till the first comma. But it is greedy enough to match all the xcharacters it came across untill last comma instead of the first comma. so it matches till the last command. and the second match is the rest of the line.

to avoid greedy pattern match adda ? after *

来源：https://stackoverflow.com/questions/15106250/how-does-pattern-matching-work-in-perl

标签

regex

perl

pattern-matching