1. Start with literal text
If you write cat, the engine searches for that sequence. This simple step teaches the core idea: a Regex describes what should appear.
Regex: cat Matches: cat, bobcat, catfish
2. Add character classes
Brackets let you choose possibilities. [aei] means one of those letters, while ranges such as [A-Z] and [0-9] represent larger sets.
Regex: b[aei]g Matches: bag, beg, big
3. Control quantity
Quantifiers describe how many times the previous token may appear. + means one or more, * means zero or more, ? makes something optional, and {n} requires an exact count.
4. Learn anchors early
^ and $ prevent a validation pattern from matching only a substring inside a larger value. Understanding anchors early prevents many false positives.
Regex: ^[A-Z]{2}\d{2}$\nMatches: AB12\nRejects: XXXAB12YYY5. Practice with feedback
After each attempt, inspect both false positives and false negatives. A good pattern is not only able to match examples you want; it also rejects similar values that break the rule.
Next step: test it for real
Use the Playground to experiment with the pattern, then practice in the game with feedback on false positives and false negatives.