Matching a string NOT preceded by different strings

Question

I want a match only if the string milk is not preceded at any time by any or all of 4 specific words (for simplicity call these words AA, BB, CC, DD).

So I went to the store and bought some milk would match but the following wouldn't:

AA went to the store and bought some milk or BBCCDDmilk

Put another way, how do I get the opposite response of: /.*?(AA|BB|CC|DD).*?milk/

I think I'm supposed to put a caret somewhere but haven't been able to figure it out. Thank you.

Answer 1

Description

This regex will validate each line in the given text to ensure it doesn't have the string {aa, bb, cc, dd} preceding the string milk on any single line. Matching lines are then returned. Note: the examples in OP show that the matched "words" are simply strings, and white space and word boundaries do not matter.

^(?!.*?(?:AA|BB|CC|DD).*?milk).*

• ^ anchor this match to the start to of the line
• (?! start negative look ahead, if my contents match successfully then I'll fail
• .*?(?:AA|BB|CC|DD).*?milk look for strings aa bb cc dd followed by string milk
• ) end the lookahead
• .* match the entire sentence

PHP Code Example:

Input Text

AA went to the store and bought some milk
BBCCDDmilk
I went to the store and bought some milk
Aardvarks like milk

Code

<?php
$sourcestring="your source string";
preg_match_all('/^(?!.*?(?:AA|BB|CC|DD).*?milk).*/im',$sourcestring,$matches);
echo "<pre>".print_r($matches,true);
?>

Matches

$matches Array:
(
    [0] => Array
        (
            [0] => I went to the store and bought some milk
        )

)