## Why is this an issue?

When using metacharacters like `\w` without the Unicode flag or when using hard-coded character classes like `[a-zA-Z]`, letters outside of the ASCII range, such as umlauts, accented letters, or letters from non-Latin languages, won’t be matched. This may cause code to incorrectly handle input containing such letters.

To correctly handle non-ASCII input, it is recommended to use the Unicode flag `\u` or Unicode character properties like `\p{L}`.

### Noncompliant code example

    preg_match("/[a-zA-Z]/", "ö"); // returns 0
    preg_match("/\w/", "ö"); // returns 0

### Compliant solution

    preg_match("/\w/u", "ö"); // returns 1
    preg_match("/\p{L}/", "ö"); // return 1