用`preg_replace_callback`利用正则表达回调的功能

preg_replace_callback 是 PHP 中用于动态字符串替换的强大工具，它通过为每个正则匹配调用自定义函数实现复杂逻辑。1. 函数语法为 preg_replace_callback($pattern, $callback, $subject)，其中 $callback 可对匹配内容进行动态处理；2. 可用于数值变换，如将 [10] 替换为 [20]；3. 支持多捕获组操作，例如将 YYYY-MM-DD 格式日期转为 “May 15, 2024”；4. 结合 use 关键字可维护状态，如为每个单词添加递增编号；5. 适用于需上下文判断、数学运算或外部数据查询的场景，是实现智能文本处理的首选方法。

When you need more control over string replacements in PHP, preg_replace_callback is a powerful tool that goes beyond simple find-and-replace operations. Unlike preg_replace, which only allows static or pattern-based substitutions, preg_replace_callback lets you execute custom logic during each replacement—making it ideal for dynamic, context-aware transformations.

What Is preg_replace_callback?

preg_replace_callback applies a regular expression to a string and, for every match found, calls a user-defined function to determine the replacement value. This callback function receives the matched segments as an array and returns the string that should replace that match.

The basic syntax is:

preg_replace_callback($pattern, $callback, $subject);

• $pattern: The regex pattern to search for.
• $callback: A callable function that processes each match.
• $subject: The input string (or array of strings).

This setup is especially useful when the replacement depends on the content of the match or requires logic like counting, transformation, or external data lookup.

Dynamic Content Transformation

One of the most common uses is transforming content based on what’s captured in the match.

For example, suppose you want to double the numeric value found inside square brackets:

$text = "The values are [10], [20], and [30].";

$result = preg_replace_callback(
    '/\[(\d )\]/',
    function ($matches) {
        $number = (int)$matches[1];
        return '[' . ($number * 2) . ']';
    },
    $text
);

echo $result; // Outputs: The values are [20], [30], and [60].

Here, the regex /\[(\d )\]/ captures digits inside square brackets. The callback converts the captured string to an integer, doubles it, and wraps it back in brackets.

This kind of dynamic manipulation isn’t possible with plain preg_replace.

Working with Multiple Capture Groups

You can also use callbacks when your pattern includes multiple capture groups and the replacement logic depends on their combination.

Say you're reformatting dates from YYYY-MM-DD to a more readable format:

$dateText = "Event on 2024-05-15 and another on 2024-06-20.";

$result = preg_replace_callback(
    '/(\d{4})-(\d{2})-(\d{2})/',
    function ($matches) {
        $year = $matches[1];
        $month = $matches[2];
        $day = $matches[3];

        $timestamp = strtotime("$year-$month-$day");
        return date('F j, Y', $timestamp); // Convert to "May 15, 2024"
    },
    $dateText
);

echo $result; // Outputs: Event on May 15, 2024 and another on June 20, 2024.

The callback uses PHP’s strtotime and date functions to produce a natural-language date, something a static replacement could never achieve.

Advanced Use: Stateful Replacements

Sometimes, you need to maintain state across multiple matches—like incrementing counters or tracking replacements.

Because the callback can access variables from the parent scope via use, you can maintain context:

$count = 0;
$text = "Items: apple, banana, cherry.";

$result = preg_replace_callback(
    '/\b\w \b/',
    function ($matches) use (&$count) {
        return strtoupper($matches[0]) . '_' . (  $count);
    },
    $text
);

echo $result; // Outputs: Items_1: apple_2, banana_3, cherry_4.

Note the use of &$count to pass the counter by reference, allowing the callback to modify it across calls.

This is perfect for tasks like numbering matches, alternating styles, or conditional logic based on match order.

Final Thoughts

preg_replace_callback unlocks a level of flexibility that plain string replacement functions can’t match. Whether you’re:

• Modifying captured values mathematically
• Reformatting dates, tags, or markup
• Applying business logic per match
• Maintaining state across replacements

…it’s the go-to function for intelligent, dynamic text processing in PHP.

Just remember:

• Keep the callback efficient, especially on large inputs.
• Use capture groups wisely to pass needed context.
• Always test edge cases—regex can be unforgiving.

With a solid pattern and a smart callback, you can transform complex text with precision.

Basically, if you're doing anything beyond literal swaps, preg_replace_callback is probably your best bet.

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