Re: How to match regex in bash? (any character)

[Peng Yu]

On Mon, Sep 26, 2011 at 9:49 PM, John Reiser <jreiser@bitwagon.com> wrote:
> Peng Yu wrote:
>> I know that I should use =~ to match regex (bash version 4).
>>
>> However, the man page is not very clear. I don't find how to match
>> (matching any single character). For example, the following regex
>> doesn't match xxxxtxt. Does anybody know how to match any character
>> (should be '.' in perl) in bash.
>>
>> [[ "$1" =~ "xxx.txt" ]]
>
> The manual page for bash says that the rules of regex(3) apply:
>
>              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 regex(3)).  The return value is 0 if 
> the
>              string matches the pattern, and 1 otherwise.
> and also:
>               Any part of the pattern may be quoted to force it to be matched 
> as a
>              string.
>
> Thus in the expression   [[ "$1" =~ "xxx.txt" ]]   the fact that the pattern
> is quoted [here the whole pattern appears within double quotes] has turned the
> dot '.' into a plain literal character, instead of a meta-character which 
> matches
> any single character.
>
> The usual method of avoiding quotes in the pattern is to omit them:
>              [[ $1 =~ xxx.txt ]]   # the dot '.' in the pattern is a 
> meta-character
> or to use a variable:
>              pattern="xxx.txt"   # a 7-character string
>              [[ $1 =~ $pattern ]]   # the dot '.' in $pattern is a 
> meta-character
> Example: using all literals in an instance of bash:
>               $ [[ xxxxtxt =~ xxx.txt ]] && echo true
>               true
>               $
>
> Also notice that quotes are not needed around the left-hand side  $1 :
>                                                                       Word 
> split‐
>              ting and pathname expansion are not performed on the  words  
> between
>              the  [[  and  ]] ...
>
> Thus there is no need to use quotation marks to suppress word splitting
> inside double brackets [[ ... ]].

Thanks for the clarifications of all the replies. Now the manual makes
much more sense to me.

-- 
Regards,
Peng