正则表达式 – 如何使用带有字符类的加号作为正则表达式的一部分

发布时间：2020-12-14 06:05:38 所属栏目：百科来源：网络整理

导读：在cygwin中,这不会返回匹配： $echo "aaab" | grep '^[ab]+$' 但这确实会返回一个匹配： $echo "aaab" | grep '^[ab][ab]*$'aaab 这两个表达式不一样吗？有没有办法表达“字符类的一个或多个字符”而不键入两次字符类(如在秒示例中)？根据this link,两个表

在cygwin中,这不会返回匹配：

$echo "aaab" | grep '^[ab]+$'

但这确实会返回一个匹配：

$echo "aaab" | grep '^[ab][ab]*$'
aaab

这两个表达式不一样吗？
有没有办法表达“字符类的一个或多个字符”而不键入两次字符类(如在秒示例中)？

根据this link,两个表达式应该是相同的,但也许Regular-Expressions.info不包括cygwin中的bash.

解决方法

grep有多个匹配的“模式”,默认情况下只使用一个基本集,除非它们被转义,否则它们不识别多个元字符.您可以将grep放入扩展或perl模式以进行评估.

从男人grep：

Matcher Selection
  -E,--extended-regexp
     Interpret PATTERN as an extended regular expression (ERE,see below).  (-E is specified by POSIX.)

  -P,--perl-regexp
     Interpret PATTERN as a Perl regular expression.  This is highly experimental and grep -P may warn of unimplemented features.


Basic vs Extended Regular Expressions
  In basic regular expressions the meta-characters ?,+,{,|,(,and ) lose their special meaning; instead use the backslashed versions ?,+,{,|,(,and ).

  Traditional egrep did not support the { meta-character,and some egrep implementations support { instead,so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.

  GNU  grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification.  For example,the command grep -E '{1' searches for the two-character string {1 instead of reporting a syntax
       error in the regular expression.  POSIX.2 allows this behavior as an extension,but portable scripts should avoid it.

或者,您可以使用egrep而不是grep -E.

（编辑：李大同）

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