php-regexHow to use backreference in PHP regex?

Backreferences in PHP regex are used to match the same text as previously matched by a capturing group.

Example code

$string = 'The quick brown fox jumps over the lazy dog.';
$pattern = '/(\w+)\s+(\w+)\s+(\w+)\s+(\w+)\s+over\s+\1/';
if (preg_match($pattern, $string, $matches)) {
    echo "Matched: {$matches[0]}\n";
    echo "1: {$matches[1]}\n";
    echo "2: {$matches[2]}\n";
    echo "3: {$matches[3]}\n";
    echo "4: {$matches[4]}\n";
}

Output example

Matched: The quick brown fox jumps over the
1: The
2: quick
3: brown
4: fox

Code explanation

• $string: This is the string that will be tested against the regular expression.
• $pattern: This is the regular expression that will be used to match the string. The backreference \1 is used to match the same text as previously matched by the first capturing group.
• preg_match(): This is the function used to match the string against the regular expression.
• $matches: This is an array that will contain the matches found by the regular expression.

Helpful links

• PHP Regular Expression Functions
• Backreferences in Regular Expressions

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