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Add warning about "missing" parts of regex #68

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Add warning about "missing" parts of regex #68

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11 changes: 10 additions & 1 deletion README.md
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Expand Up @@ -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._
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