Re: What is wrong with a regex?

[Peng Yu]

On 2/3/23, Dennis Williamson <dennistwilliamson@gmail.com> wrote:
> On Fri, Feb 3, 2023 at 9:59 PM Koichi Murase <myoga.murase@gmail.com>
> wrote:
>
>> 2023年2月4日(土) 12:44 Peng Yu <pengyu.ut@gmail.com>:
>> > $ f=row.txt; [[ $f =~ ^row([0-9]*)(\|_x)[.]txt$ ]]; echo $?
>> > 1
>>
>> You need to write (|_x) instead of (\|_x). In the conditional command
>> [[ ... ]], the character `|' loses the original meaning of the pipe
>> operator, so you can directly specify it without quoting. If you quote
>> it as \|, it becomes a regular expression that matches a literal
>> single character `|'.
>>
>> $ [[ '|' =~ \| ]]; echo $?
>> 0
>>
>>
> Bash (and grep) don't allow an empty subexpression.
>
>  f=foo; [[ $f =~ (|o) ]]; echo $?; echo "${BASH_REMATCH}"
> 2
>
> $ echo foo | grep -E '(|o)'
> grep: empty (sub)expression

So then have to split it into two regexes? If I have two | with one
branch empty, then I'd have 2x2=4 regexes. If I have n | with one
branch empty, then I'd have 2^n regexes? If so, this becomes
unmanageable. How to overcome this problem?

-- 
Regards,
Peng