Re: ?: in regular expressions.

[Dennis Williamson]

On Mon, Aug 23, 2021, 4:37 PM hancooper via <help-bash@gnu.org> wrote:

> Am trying to understand what `?:` does in regular expressions. What is it
> good for?




It's a feature of PCRE.  (quoting the MacOs pcrepattern man page) it causes
the subpattern (within the enclosing parentheses) to:

"not do any capturing, and is not counted when computing the number of any
subsequent capturing subpatterns."

So you could do alternation without capturing. Using the example from that
man page:

the ((?:red|white) (king|queen))

would capture white queen and queen given "the white queen" as input but it
wouldn't capture "white" by itself.

I advise reading a lot of quality material on PCRE and regexes in general
and practicing and trying things. Also, my advice is that it's important to
learn when to not use regexes.

regular-expressions.info is a good resource.

Note that Bash does not support PCRE.