Negative first is used in PHP regulars to match positions that do not follow specific content. ^(?!.\.jpg$).*$/ can filter non-.jpg-end file names, such as photo.png?; ^(?!.error). $/m can exclude log lines containing "error"; combined use such as ^a(?!.*b).*$/ can match strings starting with a and not containing b; common misunderstandings include missing writes.*, missing anchor points, and wrong order of multi-condition overlay. Correct combination of position anchor points and wildcard characters is the key.

Negative lookahead in regular expressions is a useful but confusing feature in PHP. Simply put, it is used to match positions that are not followed by something . This article uses several practical examples to help you understand how to use this structure in PHP ?!

Match strings that do not end with a specific word

Suppose you want to find out all file names that do not end in .jpg . At this time, you can use negative direction to eliminate it first:

 $pattern = '/^(?!.*\.jpg$).*$/';

The meaning of this writing is: start from the beginning and make sure that there is no .jpg in the end in the entire string. Suitable for filtering picture file lists, log content and other scenarios.

For example:

• photo.png ? Match successfully
• image.jpg ? will not be matched

Note: ^ and $ here are required, otherwise it may mismatch strings containing .jpg in the middle.

Exclude sentences with certain keywords

Sometimes you just want to extract text that does not contain certain words. For example, want to find all log lines that do not contain the word "error":

 $pattern = '/^(?!.*error). $/m';

After adding the m modifier, ^ and $ will work on each line, not the entire string.

Use scenarios include log analysis, filtering clean data, etc. This writing method can avoid reading all first and then filtering manually, which is more efficient.

Combination of complex conditions

If you want to match something more complicated, such as "match a string that starts with a but does not contain b", you can write it like this:

 $pattern = '/^a(?!.*b).*$/';

It means: first confirm that it starts with a , and then check whether b does not appear in the entire string. This logic is a bit confusing, but it will be clear just by disassembling it:

• ^a : The beginning must be a
• (?!.*b) : There cannot be b afterwards
• .*$ : The remaining characters are fine

This writing method is suitable for some scenarios of form verification or rule judgment.

Common misunderstandings and precautions

There are several places where negative directions are particularly prone to errors:

• Forgot to add .* , resulting in only checking if there are target characters after the current position
• No anchors ^ and $ , resulting in partial matches rather than overall matches
• The order and nesting relationship are not handled properly when multiple negative conditions are superimposed

For example:

Wrong writing method:

 /(?!jpg)/

This will only check whether the current position is followed by jpg, and will not limit the entire string.

The correct way to write it should be used together with position anchor points as mentioned earlier.

Basically that's it. Although the negative direction in PHP seems a bit tangled, just remember its core logic: "What can't appear after here", and then combine it with anchor points and wildcards to write clear and practical rules.

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