Re: What is wrong with a regex?

[Dennis Williamson]

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

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