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.
