为什么在regex模式中使用POSIX字符类会产生意想不到的结果?

角色类 [:alpha:] 表示Perl正则表达式中的alpha字符,但方括号表示 不 表示它们在正则表达式中通常所做的事情。所以你需要:

$a=~/([[:alpha:]]*)/;

perlre

POSIX字符类语法

[:class:]

也可提供。请注意 [ ] 括号是文字;它们必须始终在字符类表达式中使用。

# this is correct:
$string =~ /[[:alpha:]]/;

# this is not, and will generate a warning:
$string =~ /[:alpha:]/;

你所写的不是想象中的Perl。你能逃脱惩罚是因为你已经关机了 warnings perl

POSIX syntax [: :] belongs inside character classes in regex; marked by <-- HERE in m/([:alpha:] <-- HERE *)/ at j.pl line 4.

POSIX syntax [: :] belongs inside character classes in regex; marked by <-- HERE in m/([:alpha:] <-- HERE *)/ at j.pl line 8.

想象一下!

现在, 也会告诉你:

Illegal character in prototype for main::namecmp : $a,$b at j.pl line 3.

use warnings; use strict;

sub namecmp {
    my ($aword) = $a =~ /([[:alpha:]]*)/;
    my ($bword) = $b =~ /([[:alpha:]]*)/;
    return $aword cmp $bword;
}

print join(' ', sort namecmp split ' ', scalar <>), "\n";

因为Perl不支持这种形式的POSIX字符类(使用 [[:alpha:]] @Greg's answer )

所以

[:alpha:]

被解释为由字符组成的字符类“ a h ", " l p “和” : ".

[ahlp:] 一开始 (因为 * ),例如“ baa “匹配将返回一个空字符串。一个空字符串当然比其他字符串小,所以它们将被安排在开头。