Method testRegExp

src/scanner.ts:1089–1141

(pattern: string, flags: string)

    // https://tc39.github.io/ecma262/#sec-literals-regular-expression-literals

    private testRegExp(pattern: string, flags: string): RegExp | null {
        // The BMP character to use as a replacement for astral symbols when
        // translating an ES6 "u"-flagged pattern to an ES5-compatible
        // approximation.
        // Note: replacing with '\uFFFF' enables false positives in unlikely
        // scenarios. For example, `[\u{1044f}-\u{10440}]` is an invalid
        // pattern that would not be detected by this substitution.
        const astralSubstitute = '\uFFFF';
        let tmp = pattern;
        const self = this;

        if (flags.indexOf('u') >= 0) {
            tmp = tmp
                // Replace every Unicode escape sequence with the equivalent
                // BMP character or a constant ASCII code point in the case of
                // astral symbols. (See the above note on `astralSubstitute`
                // for more information.)
                .replace(/\\u\{([0-9a-fA-F]+)\}|\\u([a-fA-F0-9]{4})/g, ($0, $1, $2) => {
                    const codePoint = parseInt($1 || $2, 16);
                    if (codePoint > 0x10FFFF) {
                        self.throwUnexpectedToken(Messages.InvalidRegExp);
                    }
                    if (codePoint <= 0xFFFF) {
                        return String.fromCharCode(codePoint);
                    }
                    return astralSubstitute;
                })
                // Replace each paired surrogate with a single ASCII symbol to
                // avoid throwing on regular expressions that are only valid in
                // combination with the "u" flag.
                .replace(
                /[\uD800-\uDBFF][\uDC00-\uDFFF]/g,
                astralSubstitute
                );
        }

        // First, detect invalid regular expressions.
        try {
            RegExp(tmp);
        } catch (e) {
            this.throwUnexpectedToken(Messages.InvalidRegExp);
        }

        // Return a regular expression object for this pattern-flag pair, or
        // `null` in case the current environment doesn't support the flags it
        // uses.
        try {
            return new RegExp(pattern, flags);
        } catch (exception) {
            /* istanbul ignore next */
            return null;
        }
    }

    private scanRegExpBody(): string {
        let ch = this.source[this.index];