A colleague is surprised that `grep 'ab*c' data.txt` matches a line containing the string `ac`. What explains the match?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands easy
Machine-checked — no person has signed for it. This question was read against the source cited below by an automated pass, which found no contradiction. That is a weaker claim than it sounds: the same kind of process wrote the question, so it can confirm its own mistake.
Treat it as a good draft rather than as settled fact, and read the source below before you rely on it. It is not used in mock exams here — only questions a person has signed for are.
How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct * means zero or more of the preceding single character, so b may be absent altogether.
Correct. The quantifier applies to the b alone, and zero repetitions is a legitimate count.
Not correct * means any sequence of characters, exactly as it does in shell filename globbing.
Wrong. That is the shell's glob; in a regular expression the equivalent is .* — a dot quantified by a star.
Not correct * means one or more of the preceding character, and grep is being lenient.
Wrong. One or more is + in extended syntax, or \+ as a GNU extension in basic syntax.
Not correct b* is a wildcard for any word starting with b, so it also matches nothing.
Wrong. Regular expressions have no concept of words unless you ask for one with -w or a boundary escape.
Why
In a regular expression a quantifier binds to the single preceding element, so ab*c means a, then any number of b including none, then c. It therefore matches ac, abc and abbbc alike. The most common confusion on this objective is between the shell's globbing, where * stands for any run of characters, and regular expressions, where the equivalent must be written .* .
Where this comes from
- Cited
- manual page regex(7)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 103 and space the ones you get wrong.
More questions on this objective
- A user starts a new terminal window in a running graphical desktop session, which launches bash as an interactive shell that is NOT a login shell. Which file in the user's home directory does bash read in that case? machine-checked
- In bash, which history expansion re-runs the entire previous command line? machine-checked
- Which bash command prints the literal five characters $USER instead of the current user name? machine-checked
- You want /opt/bin searched for executables, after all the directories already in PATH, in the current bash session and in every command started from it. Which command does that? machine-checked
- There is a passwd manual page in section 1 (the command) and another in section 5 (the /etc/passwd file format). Which command opens the section 5 page? machine-checked
- Select the TWO true statements about shell variables and the environment in bash. machine-checked