preg_match Compilation Failed

Why This Happens

PHP's PCRE regex functions throw warnings when the pattern cannot be compiled. Common causes include missing delimiters, unescaped special characters, invalid quantifiers, or unsupported regex features. The error message usually indicates the approximate position of the problem.

The Problem

$pattern = '/user[0-9+/';
if (preg_match($pattern, $input)) { // Unclosed character class
    echo 'match';
}

The Fix

$pattern = '/user[0-9]+/';
if (preg_match($pattern, $input) === 1) {
    echo 'match';
}

Step-by-Step Fix

1. 1

   Read the error position

   The error message includes an offset number. Look at that position in your pattern to find the syntax error.

2. 2

   Check delimiters and escaping

   Ensure the pattern has matching delimiters, all special characters are properly escaped, and character classes are closed.

3. 3

   Test the regex separately

   Use an online regex tester to validate your pattern before using it in PHP code.