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Comment by ngruhn | Hacker News Reader

A recent trap for me:

Regex semantics is subtly different across languages. E.g. a{,3} matches between 0 and 3 "a" characters in Python. In JavaScript it matches the literal string "a{,3}".

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Regex is more a technique than an actual specification. It would be best to find the time to go and read an introductory book about Theory of Computation where they explain the underlying mechanism.

I always use regex101 to develop my regexes. It allows you to switch between different engines.

Honorable mention to [a-z], gotta be my favorite trap

What's the trap for this one? I can't think of any engine that parses this to mean anything other than the letters a through z.

It depends on its use, ultimately, but if your goal is to find a string of letters (a common use IMO), you'll want to use something like \p{L} to ensure you don't miss non-ASCII characters.

eta: fixed regex, I had typed \L, shared from my faulty memory.

In some common implementations if $LANG is set to certain values, it will fail to match some ASCII letters. This is because not all latin character using languages put Z last in the alphabet.

Try this (you probably need to enable and generate the locale first)

    echo y | LANG=lv_LV.UTF-8 grep '[a-z]'

Locales in general should be considered a "trap", just look at Windows CSV separator handling, etc.

Not in general, but using locales for something different than affecting presentation.

That's wild. Thanks for explaining. I had no idea this depends on the locale. Looks like I have about a million scripts to fix...

[A-z] though is a fun one though as it includes a few extra symbols between upper and lowercase.

Does it? I thought Regex are defined on character classes not on numeric ASCII values. What would a Regex do on a different encoding then?

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