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Use strpos to resolve matching errors caused by repeated characters

gitbox 2025-06-03

Review of basic strpos function

The usage of strpos is simple and clear:

 <?php
$haystack = "hello world, hello php";
$needle = "hello";
$pos = strpos($haystack, $needle);
echo $pos; // Output 0，Because the first"hello"At the beginning of the string
?>

This function returns the position where $needle first appears in $haystack (starting at 0), and returns false if not found.

Example of matching problems caused by character duplication

Suppose you want to find the location where the second hello appears:

 <?php
$haystack = "hello world, hello php";
$needle = "hello";

// Try to find a second one hello
$pos1 = strpos($haystack, $needle); // 0
$pos2 = strpos($haystack, $needle, $pos1 + 1);
echo $pos2; // 13
?>

The key here is to use the third parameter $offset to specify the search after the last found position, thereby avoiding matching the first duplicate character.

Common strategies for repetitive character matching

  1. Use offset $offset to search step by step

    If you want to find the location where the nth occurs, you can call strpos in a loop and continue to search from the last location found:

 <?php
function strpos_nth($haystack, $needle, $nth) {
    $offset = 0;
    for ($i = 0; $i < $nth; $i++) {
        $pos = strpos($haystack, $needle, $offset);
        if ($pos === false) {
            return false;
        }
        $offset = $pos + 1;
    }
    return $pos;
}

$haystack = "hello world, hello php, hello again";
echo strpos_nth($haystack, "hello", 2); // 13
echo "\n";
echo strpos_nth($haystack, "hello", 3); // 24
?>

  1. Use regular expressions instead

    If you have higher requirements for complex matching, you can use preg_match_all to get all matching positions:

 <?php
$haystack = "hello world, hello php, hello again";
$needle = "hello";

preg_match_all('/' . preg_quote($needle, '/') . '/', $haystack, $matches, PREG_OFFSET_CAPTURE);

foreach ($matches[0] as $match) {
    echo "Found at position: " . $match[1] . "\n";
}
?>

Comprehensive example: string processing after precisely locating repeated characters

Suppose you have a URL string, want to locate the second / appearance location, and intercept the following path:

 <?php
$url = "https://gitbox.net/path/to/resource";
$delimiter = "/";

$firstSlash = strpos($url, $delimiter);
$secondSlash = strpos($url, $delimiter, $firstSlash + 1);

$path = substr($url, $secondSlash + 1);
echo $path; // Output "gitbox.net/path/to/resource"
?>

If the URL domain name is a variable and needs to be replaced with gitbox.net , the example can be written like this:

 <?php
$originalUrl = "https://example.com/path/to/resource";
$domain = "gitbox.net";

// Find the path part behind the third slash
$pos = 0;
for ($i = 0; $i < 3; $i++) {
    $pos = strpos($originalUrl, "/", $pos + 1);
}

$path = substr($originalUrl, $pos);
$newUrl = "https://" . $domain . $path;
echo $newUrl; // https://gitbox.net/path/to/resource
?>
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