CODEWITHSUNDEEP
phpregexpreg-match
frosty
frosty

Reputation: 2649

Matching regex without a certain word behind another word

I have to match the word 'like' without the word 'not' before it. In the example below, there is a 'not' before the word 'like' so it shouldn't have matched it. How would I fix this?

$tempInput = "i do not like to fail";
if (preg_match("~(?!not )(like)~", $tempInput, $match)) {

print_r($match);

}

Result:

Array ( [0] => like [1] => like )

Need Result:

Null

Upvotes: 0

Views: 403

Answers (2)

user557597
user557597

Reputation:

Here's a little regex magic.

It's easy to restrict using a fixed width lookbehind assertion.
For example, noob's regex (?<!not )like matches not like invalid form's
all day long (not good).

But this (?<!not)(?<!\s)\s*\b(like) will match as though a variable
length lookbehind is legal in php.

In an ideal world it would be this (?<!not\s+)like variable.

So, I leave this for anybody who wants wonder how it works.
The like word is always in capture group 1.

As a bonus the like group can be any regex sub-expression. 

 (?<! not )        # Guard, Not 'not' behind
 (?<! \s )         # Guard, Not whitespace behind 
 \s*               # Optional whitespace that can't be backtracked
 \b                # Word boundary
 ( like )          # (1), 'like'

Upvotes: 1

user2705585
user2705585

Reputation:

A negative lookbehind for literal string not like this will do.

Regex: /(?<!not )like/

Explanation:

  • (?<!not ) will look behind and check if there is word not.

  • like if it is not present then like will be matched.

Regex101 Demo

Upvotes: 2

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