Limit validation to hexadecimal values

[click2install]

Currently, validation will accept zzzzzzzz-zzzz-zzzz-zzzz-zzzzzzzzzzzz as a valid Guid. The changes in this pull request limit validation to hexadecimal characters only.

Code and test cases are updated to reflect this change.

[gekkedev]

I'm afraid the repo is not actively maintained anymore for over a year, so your change has a low chance to be merged. There are a few forks and as maintainer of one of them I'd be happy to accept your PR there with the following changes:

1. updating dependencies - not recommended within the same PR (and in the fork already done)
   • you could remove it from this PR as well btw to increase the chances of getting it accepted
2. Increment the version in package.json and package-lock.json.
3. Due to my limited RegEx understanding I'm not sure if the few hardcoded GUIDs in the tests are enough. There is a test for generating 3000 GUIDs and validating them:
   https://github.com/NicolasDeveloper/guid-typescript/blob/2c4569d92bb60987bac9fef1d14e341654d8bba8/tests/guid.spec.ts#L63
   But: The checks in the other direction are only a few examples. What if, we create a JSON file of GUIDs (generated elsewhere) and import it in the tests to check for their validity?
   Just noticed it is not a good test: Unique GUID generation test is faulty ez-libs/ez-guid#42 - feel free to fix in the same PR

[snico-dev]

Sorry, everyone! I haven't been maintaining this repo for the past few years due to personal reasons. I'll review the requests and accept them soon. Thanks!