This is exactly it. Regex is super simple. The difficulty is maintaining a mental mapping between language/util <-> regex engine <-> engine syntax & character class names. It gets worse when utils also conditionally enable extended syntaxes with flags or options.

The hardest part is remembering whether you need to use \w or [:alnum:].

Way too few utils actually mention which syntax they use too. Most just say something accepts a "regular expression", which is totally ambiguous.