Write examples that should match and near misses that should not. Include missing fields, extra spaces, punctuation, case changes, newlines, and Unicode if the real input has them. Run all cases in a regex tester after each change.
When a new rule breaks an old example, isolate that part of the pattern and inspect anchors, greediness, groups, and escaping. Check the regex flavor used by the application; a pattern can behave differently across engines. If the expression becomes opaque, simpler parsing code may be safer.
One successful sample is evidence only for that sample.
Turn examples into a small test table: input, expected match, expected captures and reason. Include empty strings, Unicode, newlines, repeated delimiters and very long near-misses. Then check anchors and flags explicitly—^/$, multiline, dot-all, case-insensitive and global behavior change results across engines. A regex is reliable only when its non-matches are tested as carefully as its matches.
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