regex - 如何在 Perl 中将正则表达式模式定义为常量？

Question

我想在我的脚本顶部定义一个 regexp 常量，稍后使用它来检查日期字符串的格式。

我的日期字符串将被定义为

$a="2013-03-20 11:09:30.788";

但它失败了。
我应该怎么做 ？

 use strict;
 use warnings;


 use constant REGEXP1 => "/^\d{4}\D\d{2}\D\d{2}\D\d{2}\D\d{2}\D\d{2}\D\d{3}$/";

 $a="2013-03-20 11:09:30.788";
 if(not $a=~&REGEXP1){
         print "not"

 }else{
         print "yes"

 }
 print "\n";

Answer 1

首先我们来看"/^\d{4}\D\d{2}\D\d{2}\D\d{2}\D\d{2}\D\d{2}\D\d{3}$/";如果你有警告，你应该得到:

Unrecognized escape \d passed through at C:\temp\jj.pl line 7.Unrecognized escape \D passed through at C:\temp\jj.pl line 7.Unrecognized escape \d passed through at C:\temp\jj.pl line 7.…

If you print the value of REGEXP1, you will get /^d{4}Dd{2}Dd{2}Dd{2}Dd{2}Dd{2}Dd{3} (*wait, what happened to $/?). Clearly, that does not look like the pattern you wanted.

Now, you could type "/^\\d{4}\\D\\d{2}\\D\\d{2}\\D\\d{2}\\D\\d{2}\\D\\d{2}\\D\\d{3}\$/" and then interpolate that string into a pattern, but that's too much work. Instead, you can define your constant using the regexp quote operator, qr:

#!/usr/bin/env perl

use 5.012;
use strict;
use warnings;

use constant REGEXP1 => qr/^\d{4}\D\d{2}\D\d{2}\D\d{2}\D\d{2}\D\d{2}\D\d{3}$/;

my $s = "2013-03-20 11:09:30.788";

say $s =~ REGEXP1 ? 'yes' : 'no';

还有一个问题: \d和 \D将匹配的不仅仅是 [0-9]和 [^0-9] ， 分别。因此，相反，您可以编写如下模式:

use constant REGEXP1 => qr{
    \A
    (?<year>  [0-9]{4} ) -
    (?<month> [0-9]{2} ) -
    (?<day>   [0-9]{2} ) [ ]
    (?<hour>  [0-9]{2} ) :
    (?<min>   [0-9]{2} ) :
    (?<sec>   [0-9]{2} ) [.]
    (?<msec>  [0-9]{3} )
    \z
}x;

但是，您仍然面临这些值是否有意义的问题。如果这很重要，您可以使用 DateTime::Format::Strptime .

#!/usr/bin/env perl

use 5.012;
use strict;
use warnings;

use DateTime::Format::Strptime;

my $s = "2013-03-20 11:09:30.788";

my $strp = DateTime::Format::Strptime->new(
    pattern => '%Y-%m-%d %H:%M:%S.%3N',
    on_error => 'croak',
);

my $dt = $strp->parse_datetime($s);
say $strp->format_datetime($dt);