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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
craaaxy hook 181818

and wanting to match all lines that do not contain hook:

$ grep -v hook test.txt

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

The -P means use perl-style regex syntax

  • The ?: is for clustering - breaking the source into groups and then running the expression over it