Using grep with basic regular expressions and POSIX character class syntax, which pattern matches any line containing at least one decimal digit?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands medium
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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 grep '[[:digit:]]' file
Correct. [:digit:] is the class name and it must sit inside its own bracket expression, which is why two levels of brackets appear.
Not correct grep '[:digit:]' file
Wrong. The class name has to sit inside a bracket expression of its own, and the outer pair is missing here. GNU grep recognises the mistake and refuses the pattern outright with "character class syntax is [[:space:]], not [:space:]", exiting with an error instead of searching.
Not correct grep '\d' file
Wrong. \d is Perl syntax. POSIX basic and extended regular expressions have no such escape; GNU grep only understands it with -P.
Not correct grep '[[digit]]' file
Wrong. This is a bracket expression matching [, d, i, g or t, followed by a literal ], which is nothing like a digit class.
Why
A POSIX character class is written [:name:] and is only meaningful inside a bracket expression, hence the doubled brackets in [[:digit:]]. The classes worth knowing are alpha, digit, alnum, upper, lower, space, punct and xdigit. Their advantage over ranges such as [0-9] or [a-z] is that they stay correct under other locales and collation orders.
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.
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