Regular expressions are easier to trust when they have been tested against realistic text.
Instead of writing a pattern directly into code and rerunning tests repeatedly, validate it with sample input first.

When this helps

Suppose you need to extract request IDs from logs.

2026-05-17T10:15:31Z INFO request_id=req_8f3a2 user_id=42 status=200
2026-05-17T10:15:32Z ERROR request_id=req_91bc0 user_id=42 status=500

Start with a pattern like this.

request_id=(req_[a-z0-9]+)

Then check whether the captured value is exactly what your code needs.

Add negative examples

Do not test only the happy path.
Add examples that should not match so the pattern does not become too broad.

request_id=req_8f3a2
request_id=
request=req_999
request_id=REQ_ABC

This helps catch issues around empty values, similar key names, and case sensitivity before the pattern reaches production code.

Useful before replacements

Regex validation is especially important before masking or replacing text.
If the match range is too broad, a replacement can remove more data than intended.
Confirm the match first, then move the pattern into code.