ITPEC FE Morning October 2018 Question 5

Regular Expressions (Pattern Matching) — understanding how regex quantifiers define valid string sets.

The pattern [A-Z]+[0-9]* breaks down as:
- [A-Z]+ — one or more uppercase letters (A–Z)
- [0-9]* — zero or more digits (0–9)

A valid string must consist of only uppercase letters followed optionally by digits, with no other characters allowed.

Checking each option:
- ABCDEF → 6 uppercase letters, 0 digits → matches [A-Z]+[0-9]* ✓
- 456789 → starts with digits, no leading letters → fails [A-Z]+
- ABC+99 → contains literal + character, which is neither [A-Z] nor [0-9]
- ABC99* → contains literal * character, which is neither [A-Z] nor [0-9]

Why not others:
- (a) 456789 has no uppercase letters at the start; [A-Z]+ requires at least one
- (b) ABC+99 includes the character + which is not part of [A-Z] or [0-9]
- (c) ABC99* includes the character * which is not part of [A-Z] or [0-9]

Key rule: + means one or more, * means zero or more — and the regex must match the entire string with no extra characters.

AI-generated — may contain errors