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ISRO CS 2025 Q.28: solution and answer

ISRO 2025 · CSQ.28 · 1 mark · MCQ

Which of the following hexadecimal constants match the regular expression given? (0x[0-9A-F]+)

  1. (A)

    0x4C5

  2. (B)

    0x50b

  3. (C)

    04A

  4. (D)

    0x

See the question in the paper
ISRO CS 2025 question 28Tap to open full size

Answer and solution

ANSWER

Only option (A), 0x4C5, matches the expression.

Official answer: ASource: ISRO's final answer key of 15 May 2026

  1. The expression 0x[0-9A-F]+ is the literal prefix 0x, followed by one or more characters, each of which is a digit 0-9 or an uppercase letter A-F.

  2. The character class [0-9A-F] is case-sensitive, so lowercase letters a-f are not in it.

  3. The + requires at least one character after the prefix, so the prefix alone is not enough.

  4. (a) 0x4C5: prefix 0x, then 4, C, 5, all in the class. It matches.

  5. (b) 0x50b: the last character b is lowercase, so it is not in [0-9A-F]. It does not match.

  6. (c) 04A: it begins with 04, not the literal 0x. It does not match.

  7. (d) 0x: nothing follows the prefix, so + is not satisfied. It does not match.

  8. Only (a) matches.

KEY IDEA

Read a regular expression piece by piece: literal prefix, character class (case-sensitive), then the quantifier + meaning at least one character.

WATCH OUT FOR

Treating [0-9A-F] as if it also accepted lowercase hex digits (so choosing 0x50b), or accepting the bare prefix 0x although + demands at least one digit.

From the ISRO CS 2025 paper. Every question of this paper · The full paper (PDF, ISRO) · All 5 ISRO Finite automata questions, with notes

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