regex - 如何使用 bash(3.2.25) 的一个额外的二元运算符 :=~

Question

看了rhel5.5的bash(3.2.25)的资料，对"=~"怎么用感到困惑

# match the IP, and return true
[kevin@server1 shell]# [[ 192.168.1.1 =~ "^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$" ]] && echo ok || echo fail
ok

# add double qoute
# return false, en ... I know.
[kevin@server1 shell]# [[ 192.168 =~ "^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$" ]] && echo ok || echo fail
fail

# remove double qoute
# return ture ? Why ?
[kevin@server1 shell]# [[ 192.168 =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]] && echo ok || echo fail
ok 

那么，我应该把字符串放在运算符右边吗？
为什么第二个命令返回true，按理说应该返回false!

信息如下:

An additional binary operator, =~', is available, with the same
precedence as==' and !='. When it is used, the string to the
right of the operator is considered an extended regular expression
and matched accordingly (as in regex3)). The return value is 0 if
the string matches the pattern, and 1 otherwise. If the regular
expression is syntactically incorrect, the conditional
expression's return value is 2. If the shell optionnocasematch' (see the description of shopt' in *Note Bash Builtins::) is
enabled, the match is performed without regard to the case of
alphabetic characters. Substrings matched by parenthesized
subexpressions within the regular expression are saved in the
array variableBASH_REMATCH'. The element of BASH_REMATCH' with
index 0 is the portion of the string matching the entire regular
expression. The element ofBASH_REMATCH' with index N is the portion of the string matching the Nth parenthesized subexpression.

Answer 1

推荐的、最广泛兼容的处理正则表达式模式的方法是用单引号单独声明它们:

$ re='^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$'
$ [[ 192.168.1.1 =~ $re ]] && echo ok || echo fail 
ok
$ [[ 192.168 =~ $re ]] && echo ok || echo fail 
fail

可以在 Greg's Wiki 上找到关于 bash 版本行为差异的一些讨论。 - 关键信息是使用不带引号的变量是最好的方法。