expr1 : expr2
         The ``:'' operator matches expr1 against expr2,which must be a
         regular expression.  The regular expression is anchored to the
         beginning of  the string with an implicit ``^''.  expr expects
         "basic" regular expressions,see re_format(7) for more informa-
         tion on regular expressions.

@只是@,因为它在正则表达式中没有特殊含义.从而,

expr _@foo@ : "_@[^@]*@"

将成功,并输出6(它是匹配字符的数量);而

expr _x@foo@ : "_@[^@]*@"

将输出0并返回$？中的失败代码,因为它无法匹配任何内容.

如果您不熟悉正则表达式,则示例中给出的是：下划线(_)后跟两个符号(@)夹着任意数量的非符号字符.