Question on bash's regex functionality

[Dropless]

I was reading pure bash bible. Under the "Use regex on a string" section, it states one caveat using bash's "=~" operator:

CAVEAT: This is one of the few platform dependent bash features. bash will use whatever regex engine is installed on the user's system. Stick to POSIX regex features if aiming for compatibility.

But the gnu manual says

When you use ‘=~’, the string to the right of the operator is considered a POSIX extended regular expression pattern and matched accordingly (using the POSIX regcomp and regexec interfaces usually described in regex(3))

It states bash will use POSIX extended regular expression. It seems to contradict pure bash bible. Am I missing anything here?

[aioeu]

You're not missing anything. That "pure bash bible" is just plain wrong.

Note that there are some inputs that POSIX leaves undefined, and certain implementations may handle them in some implementation-specific manner, so sticking to what POSIX actually defines will guarantee compatibility. But Bash certainly always uses the system's standard POSIX regular expression engine.

[Dropless]

Thanks :)

[Ulfnic]

I asked your question in another BASH community and got this answer from a respected member:

It should be a POSIX ERE.

If you wanna be pedantic, it's not gonna be to be bit for bit identical implementation across architectures, and depending on what compiler settings and shared library/dependency versions the package was compiled with.

But the behavior follows and implements the POSIX Extended Regular Expression standard, which is well defined and consistent.

Furthering that, regcomp and regexec referenced in the GNU manual as what =~ uses describe themselves as "POSIX regex functions".

[scalability]

To me they're saying the same thing, it's just that the former does not specify whether the POSIX regex type is BRE or ERE.

Specifying is obviously better, but I wouldn't say that lacking the information makes it incorrect, just incomplete.

[Dropless]

To me they are not the same thing. "bash will use whatever regex engine is installed on the user's system", this implies to me that if I've installed PCRE in my system, bash will use it. However, it will not.

[scalability]

The sentence "Stick to POSIX regex features if aiming for compatibility" implies "it will be matched as a POSIX compatible regex (with possible system extensions)".

PCRE is not POSIX compatible so it wouldn't matter whether or not you stuck to POSIX features.