Regular expression to capture a selector within a stylesheet

css-selector-regex.php

<?php

/**
 * Creates a regular expression string for properly capturing a specific CSS
 * selector within a stylesheet. This regular expression ensures that the
 * captured selector isn't actually a substring of another selector.
 *
 * As always, regular expressions should be considered a last resort for most
 * string manipulation due to their high probablity of human error. Think hard
 * about whether or not this solution is truly the last or best one available to
 * you before employing it.
 *
 * This regular expression can be broken down into and explained in three parts:
 * 1. A positive lookbehind group for the character preceding the selector.
 * 2. A capture group for the selector itself.
 * 3. A positive lookahead group for the character following the selector.
 *
 * ### First group (positive lookbehind) ###
 * Requires that the character preceding tag be:
 * - Start of file (^), or
 * - Any form of whitespace (\s), or
 * - A preceding rule's closing brace (}), or
 * - A child (>), adjacent sibling (+), or general sibling (~) combinator
 *
 * ### Second group (capturing) ###
 * Identifies the exact tag to search for.
 *
 * ### Third group (positive lookahead) ###
 * Requires that the character following tag be:
 * - A comma indicating the next rule (,), or
 * - Any form of whitespace (\s), or
 * - This rule's opening brace ({), or
 * - An ID (#), class (.), or attribute ([) selector, or
 * - A child (>), adjacent sibling (+), or general sibling (~) combinator, or
 * - A pseudo-selector (:)
 *
 * Tests for this Regex are available at https://regex101.com/r/X1mqxs/3.
 */
function create_css_selector_regex(string $selector): string
{
    return "/(?<=^|\s|}|>|\+|~)($selector)(?=,|\s|{|#|\.|\[|>|\+|~|:)/";
}

preg_replace(create_css_selector_regex('img'), 'custom-img');