add(value = '') {
        this._source += value;
        const pattern = this._prefixes + this._source + this._suffixes;

        this.compile(pattern, this._modifiers);

        return this;
    }
Add stuff to the expression and compile the new expression so it's ready to be used.
@param {(string|number)} [value=''] stuff to add
@returns {VerbalExpression} recompiled instance of VerbalExpression
@memberof VerbalExpression