Write a regular expression to match lines not containing a word

David Y.

February 15, 2023

The Problem

How can I write a regular expression to match lines not containing a specific word?

The Solution

You can do this using a negative lookahead assertion. The PCRE regular expression below will match any line that does not contain the word “word”:

/^((?!word).)*$/gm
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Here’s what it does:

/: Start regular expression.
^: Match the beginning of a line.
(: Start capture group 1.
(?!word): Negative lookahead assertion. If “word” is found, discard the current match, otherwise continue evaluating the expression.
.: Match any character.
): End capture group 1.
*: Match 0 or more of capture group 1.
$: Match the end of the line.
/: End regular expression
g: Global flag. Find all matches in the string, not just the first one.
m: Multiline flag. Use ^ and $ to match the start and end of a line, rather than the start and end of the whole string.

Notably, this expression will not match empty lines. If this behavior is desired, we can add the s (dotall) flag, which will make . match all characters including newlines:

/^((?!word).)*$/gms
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If the s flag is not supported by your regex implementation, you can replace . with [\s\S] (\s matches whitespace characters, and \S matches everything except whitespace characters).