Ignoring whitespaces in Javascript RegExp patterns

ignoreWhitespaces.js

/** 
 * ignores comments and whitespaces in regexp patterns,
 * similar to RegexOptions.IgnorePatternWhitespace in C#
 * or Pattern.COMMENTS in Java.
 * @example
 * // returns /[a-z][0-9]+/i
 * new RegExp(ignoreWhitespaces`
 *     [a-z]   # any latin letter
 *     [0-9]+  # any number
 * `, 'i');
 */
export function ignoreWhitespaces(/** @type TemplateStringsArray */ pattern, /** @type string[] */ ...expr) {
    let raw = '';
    for (let i = 0; i < pattern.raw.length; i++) {
        raw += pattern.raw[i];
        if (i < expr.length) {
            raw += expr[i]; // allow dynamic patterns, e.g. `a${'b'}c`
        }
    }
    let result = '';
    for (let line of raw.split('\n')) {
        const commentIndex = line.search(/(?<!\\)#/); // skip comments, but allow escaping with \
        if (commentIndex >= 0) {
            line = line.substring(0, commentIndex);
        }
        result += line.trim();
    }
    return result;
}