103.7Basic
Extract only color and colour from words.txt.
The u may occur zero or one time.
Which ERE is correct?
× Incorrect
In an ERE, ? makes the preceding element optional, allowing zero or one occurrence.
Detailed explanation
Incorrect
^colou*r$Incorrect. * permits any number of u characters, not just zero or one.
Correct
^colou?r$Correct. u? allows color and colour.
Incorrect
^colou+r$Incorrect. + requires at least one u, so color is excluded.
Incorrect
^colo(u|r)$Incorrect. The alternation changes the word structure and does not represent optional u.
Try it yourself
An example you can run in a temporary verification environment.
printf '%s\n' color colour colouur colr | grep -E '^colou?r$'Expected result
color
colourKey points
- ? means zero or one
- It applies to the preceding element
- Anchors exclude extra characters
Notes
- Environment: GNU grep 3.x / LC_ALL=C
- Command output formatting can vary slightly by distribution or tool version.
- Run the example in a temporary directory or process when possible.
Foundation review
Read the scope first
Check whether the command acts on the current shell, a new process, an existing process, or a file.
Verify the observable result
Use the supplied command and compare the output with the expected result.
Question IDKP-103-105
EnvironmentGNU grep 3.x / LC_ALL=C
Last technical review2026-08-12
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