\x is the character null \0 not hexadecimal
should use hexadecimal \h instead
or [a-fA-F0-9] (if you want to be explicit as \h is horizontal whitespace in some regex engines)

here's my VSCode extension to help with authoring TextMate grammars https://marketplace.visualstudio.com/items?itemName=RedCMD.tmlanguage-syntax-highlighter