regex - 匹配空格但不匹配换行符

Question

我有时想匹配空格而不是换行符。

到目前为止，我一直在使用[\t]。有没有不那么尴尬的方法？

Answer 1

使用双重否定:

/[^\S\r\n]/

也就是说，非非空白(大写的 S 补足)或非回车或非换行。将外部 not(即，字符类中的补码 ^)分配为 De Morgan's law ，这相当于“空格但不包含回车符或换行符”。在模式中包含 \r 和 \n 可以正确处理所有 Unix (LF)、经典 Mac OS (CR) 和 DOS-ish (CR LF) newline conventions 。

无需相信我的话:

#! /usr/bin/env perl

use strict;
use warnings;

use 5.005;  # for qr//

my $ws_not_crlf = qr/[^\S\r\n]/;

for (' ', '\f', '\t', '\r', '\n') {
  my $qq = qq["$_"];
  printf "%-4s => %s\n", $qq,
    (eval $qq) =~ $ws_not_crlf ? "match" : "no match";
}

输出:

" "  => match"\f" => match"\t" => match"\r" => no match"\n" => no match

Note the exclusion of vertical tab, but this is addressed in v5.18.

Before objecting too harshly, the Perl documentation uses the same technique. A footnote in the “Whitespace” section of perlrecharclass reads

Prior to Perl v5.18, \s did not match the vertical tab. [^\S\cK] (obscurely) matches what \s traditionally did.

The same section of perlrecharclass also suggests other approaches that won’t offend language teachers’ opposition to double-negatives.

Outside locale and Unicode rules or when the /a switch is in effect, “\s matches [\t\n\f\r ] and, starting in Perl v5.18, the vertical tab, \cK.” Discard \r and \n to leave /[\t\f\cK ]/ for matching whitespace but not newline.

If your text is Unicode, use code similar to the sub below to construct a pattern from the table in the aforementioned documentation section.

sub ws_not_nl {
  local($_) = <<'EOTable';
0x0009        CHARACTER TABULATION   h s
0x000a              LINE FEED (LF)    vs
0x000b             LINE TABULATION    vs  [1]
0x000c              FORM FEED (FF)    vs
0x000d        CARRIAGE RETURN (CR)    vs
0x0020                       SPACE   h s
0x0085             NEXT LINE (NEL)    vs  [2]
0x00a0              NO-BREAK SPACE   h s  [2]
0x1680            OGHAM SPACE MARK   h s
0x2000                     EN QUAD   h s
0x2001                     EM QUAD   h s
0x2002                    EN SPACE   h s
0x2003                    EM SPACE   h s
0x2004          THREE-PER-EM SPACE   h s
0x2005           FOUR-PER-EM SPACE   h s
0x2006            SIX-PER-EM SPACE   h s
0x2007                FIGURE SPACE   h s
0x2008           PUNCTUATION SPACE   h s
0x2009                  THIN SPACE   h s
0x200a                  HAIR SPACE   h s
0x2028              LINE SEPARATOR    vs
0x2029         PARAGRAPH SEPARATOR    vs
0x202f       NARROW NO-BREAK SPACE   h s
0x205f   MEDIUM MATHEMATICAL SPACE   h s
0x3000           IDEOGRAPHIC SPACE   h s
EOTable

  my $class;
  while (/^0x([0-9a-f]{4})\s+([A-Z\s]+)/mg) {
    my($hex,$name) = ($1,$2);
    next if $name =~ /\b(?:CR|NL|NEL|SEPARATOR)\b/;
    $class .= "\\N{U+$hex}";
  }

  qr/[$class]/u;
}

<小时/>

其他应用

双否定技巧对于匹配字母字符也很方便。请记住，\w 匹配“单词字符”、字母字符以及数字和下划线。我们丑陋的美国人有时想把它写成，比如说，

if (/[A-Za-z]+/) { ... }

但是双负字符类可以尊重语言环境:

if (/[^\W\d_]+/) { ... }

这种方式表达“一个单词字符，但不是数字或下划线”有点不透明。 POSIX 字符类更直接地传达意图

if (/[[:alpha:]]+/) { ... }

或者使用 Unicode 属性，如 szbalint 建议

if (/\p{Letter}+/) { ... }