Grep Regex Invert and Lookahead
Grep Regex Invert Match and Lookahead#
Invert Match#
When one wants to match lines not containing a specific pattern you can use -v:
-v, --invert-match
Selected lines are those not matching any of the specified patterns.
Given a text file:
xxxxhhods osd ds hook sdkjchslkdfjnscasd
ab..............abc..............abc
craaaxy hook 181818
329847209fwfkjsdnfasdf
and wanting to match all lines that do not contain hook:
$ grep -v hook test.txt
ab..............abc..............abc
329847209fwfkjsdnfasdf
Regex Look Ahead#
However there may be situations where the -v flag cannot be used.
In those instances a negative lookahead can be used within the regex expression.
A lookahead means followed by - it means how it is said. It looks ahead. A negative lookahead means you want to match something that is not followed by a certain pattern.
A negative lookahead is a question mark followed by a exclamation: (?!xxx)
A positive lookahead is a question mark followed by a equals sign: (?=xxx)
So to achieve the same result as above, excluding the hook matches this can be used:
grep -P '^(?:(?!hook).)*$' test.txt
ab..............abc..............abc
329847209fwfkjsdnfasdf
The -P means use perl-style regex syntax
- The
?:is for clustering - breaking the source into groups and then running the expression over it