Regex results in “Delimiter must not be alphanumeric or backslash” [duplicate]

Answer 1

you must put pattern delimiters at the begining and at the end of the pattern, example:

$pattern = '#class="(.+)"(.+)<a.+>(.+)</a>#';

Here # is a better choice than / because you avoid to escape all the slashes inside your pattern, but you can write:

$pattern = '/class="(.+)"(.+)<a.+>(.+)<\/a>/';

As an aside comment, your pattern will cause many backtracks:

$pattern = '~class="([^"]+)"([^>]*>)<a[^>]+>([^<]+)</a>~';

will work better.

Keep in mind that + and * are by default greedy quantifiers (i.e. they take all they can).

If I use a restricted character class instead of the dot, I can stop the greediness of the quantifiers, example

[^"]+ take all characters except ", thus it stop when a " is find.

Demo:

<?php
function a($menu_item, $remove_link) {
    //$pattern = '~class="(.+)"(.+)<a.+>(.+)</a>~';
    $pattern = '~class="([^"]+)"([^>]*>)<a[^>]+>([^<]+)<\/a>~';
    if($remove_link) {
        return preg_replace($pattern, 'class="$1 selected"$2$3', $menu_item);
    }
    return $menu_item;
}

$menu_item = '<li class="menuitem first"><a href="index.php">Home</a></li>';
echo a($menu_item, true);