Forward-first assertion is implemented in PHP via (?=...) to match content that is followed by a specific pattern without capturing that part. 1. The syntax format is pattern(?=assertion), such as run(?=ing) matches the run followed by ing; 2. When using it, please note that the regular expression must be wrapped with slashes, and the assertion position should be immediately after the matching content; 3. Common errors include forgetting slashes, assertion position errors, and misunderstandings and not capturing characteristics; 4. Application scenarios include extracting the content before a specific suffix, avoiding mismatch, combining negative assertions, etc.; 5. Use it in conjunction with functions such as preg_match or preg_replace to improve the ability to process complex text.

If you want to know how to implement "positive lookahead" in PHP using regular expressions, it is actually similar to ordinary regular syntax, but the writing method is a bit special. PHP uses Perl-compatible regular expressions (PCRE), so its syntax is basically the same as that of other languages.

The following sections will explain how to write, when to use it, and common errors, to help you use positive lookahead more smoothly in PHP.

What is a positive first assertion?

The forward assertion means: the prerequisite for matching a pattern is to be followed by another specific pattern, but this "after" will not be included in the final result.

For example: If you want to find all the words run followed by ing , you can write it like this:

 run(?=ing)

This means that only matches run followed by ing , for example, the first three letters run will be matched in running , but will not be ing in the result.

In PHP, this writing method can be directly used in functions such as preg_match or preg_replace .

How to write positive assertions in PHP

PHP regular functions all start with preg_ , such as preg_match_all() . When using forward-first assertions, be careful to be correct in the format, especially in the closing order of grouping and brackets.

Sample code:

 $text = "The running dog is jumping.";
$pattern = '/run(?=ning)/';

preg_match_all($pattern, $text, $matches);
print_r($matches[0]);

Output:

 Array
(
    [0] => run
)

Note: run in running successfully matches here because it is ning afterwards.

Some points to note:

• The positive assertion must be written in the form (?=...) .
• It does not capture content itself, which means it will not appear in the $matches result.
• If you use a capture group, remember to distinguish (?:...) from a non-capture group, and (...) from a capture group.

Common usage scenarios and techniques

1. Extract content with specific suffix
   For example, you want to extract the main name part (excluding the extension) in all file names ending in .jpg :

    /(\w )(?=\.jpg)/

2. Avoid mismatching certain situations
   For example, you only want to match the px unit after the number, not other units:

    /\d (?=px)/

3. Use with negative assertions
   For example, if you want to find run that is not ing closely:

    /run(?!ing)/

   This is the negative assertion, which is exactly the opposite of the positive.

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   Where to make mistakes

   • Forgot to add slash wrapping characters : PHP regular must be used / wrapped, otherwise an error will be reported.
   • The assertion position is misplaced : Be sure to make sure the assertion follows the part you want to match.
   • The meaning of "not capture" is misunderstood : although not captured, it will affect whether it can match success.
   • The logic is confusing when multiple assertions are superimposed : for example, when two (?=...) are used at the same time, the order does not affect it, but the logic must be clear.

   ---

   Basically that's it. Once you have mastered the forward assertion, you will find it useful when dealing with complex text structures, especially when parsing HTML, CSS, or log files. As long as you practice the actual usage a few more times, you won’t be easily confused.

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