What does the warning "REG_EPAREN" in PHP indicate when using the eregi_replace function?

The warning "REG_EPAREN" in PHP indicates that there is a missing closing parenthesis in the regular expression pattern used in the eregi_replace function. To solve this issue, you need to ensure that all opening and closing parentheses in the regular expression pattern are properly matched.

// Incorrect regular expression pattern causing REG_EPAREN warning
$pattern = "/(abc/";

// Corrected regular expression pattern
$pattern = "/(abc)/";

// Using eregi_replace with the corrected pattern
$result = eregi_replace($pattern, "def", $input_string);