Add warning about "missing" parts of regex

As someone new to bash, but familiar with regular expressions, it wasn't clear to me that something like \w would not work. This added admonition should hopefully save future on-lookers the same trouble I experienced.

Author: TurekBot

README.md

@@ -292,7 +292,16 @@ An error is displayed when used simultaneously.
 #### Regular expression matching
 
 Regular expression matching can be enabled with the `--regexp` option (`-e` for short).
-When used, the assertion fails if the *extended regular expression* does not match `$output`.
+When used, the assertion fails if the *[extended regular expression]* does not match `$output`.
+
+[extended regular expression]: https://en.wikibooks.org/wiki/Regular_Expressions/POSIX-Extended_Regular_Expressions
+
+> [!IMPORTANT]  
+> Bash [doesn't support](https://stackoverflow.com/a/48898886/5432315) certain parts of regular expressions you may be used to:
+>  * `\d` `\D` `\s` `\S` `\w` `\W` — these can be replaced with POSIX character class equivalents `[[:digit:]]`, `[^[:digit:]]`, `[[:space:]]`, `[^[:space:]]`, `[_[:alnum:]]`, and `[^_[:alnum:]]`, respectively.  (Notice the last case, where the `[:alnum:]` POSIX character class is augmented with underscore to be exactly equivalent to the Perl `\w` shorthand.)
+>  * Non-greedy matching. You can sometimes replace `a.*?b` with something like `a[^ab]*b` to get a similar effect in practice, though the two are not exactly equivalent.
+>  * Non-capturing parentheses `(?:...)`. In the trivial case, just use capturing parentheses `(...)` instead; though of course, if you use capture groups and/or backreferences, this will renumber your capture groups.
+>  * Lookarounds like `(?<=before)` or `(?!after)`. (In fact anything with `(?` is a Perl extension.)  There is no simple general workaround for these, though you can sometimes rephrase your problem into one where lookarounds can be avoided.
 
 > _**Note**:
 > The anchors `^` and `$` bind to the beginning and the end of the entire output (not individual lines), respectively._