Using GNU grep, which THREE characters have their special meaning only when preceded by a backslash in a BASIC regular expression, but are special as typed in an EXTENDED one?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands hard
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The options
Choose 3.
Correct +
Correct. In a BRE you must write \+ for 'one or more'; with grep -E a bare + already means that.
Correct ?
Correct. In a BRE \? means 'zero or one'; a plain ? is a literal question mark. In an ERE ? is the quantifier directly.
Not correct *
Wrong. The asterisk means 'zero or more of the preceding item' in both BRE and ERE without any backslash.
Not correct .
Wrong. The dot matches any single character in both BRE and ERE; escaping it as \. is how you get a literal dot in either.
Correct |
Correct. Alternation is written \| in a BRE and | in an ERE.
Why
BRE and ERE share . * [ ] ^ $ as metacharacters. What differs is + ? { } ( ) | — these need a backslash in a BRE (grep, sed by default) and are special as typed in an ERE (grep -E, egrep, sed -E). POSIX only defines \{ \} and \( \) for BRE; \+ \? \| are GNU extensions, which is another reason reaching for -E is usually clearer than accumulating backslashes.
Where this comes from
- Cited
- LPI exam objective 103.7
- What it says
- Distinguish basic from extended regular expression syntax.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 103 and space the ones you get wrong.