Assume a list of soccer scores are given with the first team's score, followed by a full stop character, followed by the second team's score (e.g. 4.2). We wish to find all scores representing a draw (i.e. both teams have an equal score). Which of the following regular expressions would we use:

Question

Assume a list of soccer scores are given with the first team's score, followed by a full stop character, followed by the second team's score (e.g. 4.2).  We wish to find all scores representing a draw (i.e. both teams have an equal score).  Which of the following regular expressions would we use:

BlackTom AI · Accepted Answer

(\d)\.\1 uses three core regex features: a digit class (\d), a capture group, escaping the dot (\.) to mean a literal period, and a backreference (\1) to require the same text matched by the capture group a second time.

1) Relevant knowledge points and concepts
- \d matches any single digit (0–9).
- Parentheses ( ) create a capture group; the text matched inside can be referenced later.
- A plain . in regex matches any character, so to match a literal period you must escape it as \.
- Backreferences (\1, \2, ...) match exactly what was captured by the corresponding group.

2) Step-by-step reasoning (matching "4.4" and rejecting "4.2")
Pattern: (\d)\.\1
- (\d) captures one digit and stores it in group 1. For "4.4", group 1 = "4".
- \. matches the literal '.' between the scores.
- \1 requires the exact same content as group 1 to appear after the dot. Since group 1 = "4", \1 matches "4".
Thus "4.4" matches fully. For "4.2":
- (\d) captures "4", \. matches '.', but \1 expects "4" and the input has "2", so the pattern does not match. Therefore only draws with identical single-digit scores are matched.

3) Intermediate matching steps (example "7.7")
- (\d) -> captures "7" (group1 = "7")
- \.   -> matches the period "."
- \1   -> matches "7" (same as group1)
Match succeeds.

4) Why this is correct
The expression enforces "same digit, literal dot, same digit" by capturing the first digit and requiring the second to be identical via backreference. Escaping the dot ensures it separates the two numeric fields rather than matching any character.

5) Why common alternatives are incorrect
- \d\.\d: matches any single-digit.score pair (including non-draws like "3.2") because it does not enforce equality.
- (\d)\.\2: invalid here because \2 refers to a non-existent second group; it won’t enforce equality to the first group.
- (\d+)\.\1: would require the entire multi-digit capture to repeat (and could work for multi-digit draws like "10.10"), but it’s different from the single-digit pattern given and was not the selected answer.

6) Conclusion
The provided escaped-string "(\d)\.\1" corresponds to the regex (\d)\.\1 and correctly matches draw scores like "4.4" while rejecting non-draws; this confirms the given answer.

类似问题

Assume all University of Melbourne subject codes contain four capital letters followed by five numbers. A regular expression for finding subject codes is:

L(r) = { w E { a , b }* : w starts and ends with the same symbol } r = ? Pick all correct answers.

Pick all true statements about REGEXs:

L(r) = { w E { a , b }* : w contains at least one a } r = ? Pick all correct answers:

What language does the following REGEX r represent? r = ( lam* )* Note: phi is used for 𝜙 , and lam is used for 𝜆

REGEX is a mathematical tool to represent "some formal languages".

REGEXs represent "some formal languages" in a more compact way.

L(r) = { w E { a , b }* : w ends with ab OR starts with ab } r = ? Pick all correct answers:

更多留学生实用工具

智能学习助手

风格化写作助手

论文查重助手

文献引用助手

课堂转译助手

课堂笔记助手

Quiz搜索助手

学校历年真题

智能刷题助手

智能匹配练习

希望你的学习变得更简单