const make_pattern_builder = function() {
    let chars = [];
    let tags = [];
    return {
        add_chars: function(c) {chars.push(c);},
        add_tag: function(t) {tags.push(`<(?: *)${t}(?: *)>[^<]*<(:? *)/(?: *)${t}(?: *)>`);},
        add_autoclosing_tag : function (t) { tags.push(`<(?: *)${t}(?: *?/? *?)>`) ; },
        get_raw: function() {
            const c = [ `[${chars.join('')}]` ];
            const all = c.concat(tags);
            return all.join('|');
        },
        build: function() {
            return new RegExp(this.get_raw(), "g");
        },
        build_match_all: function() {
            return new RegExp(this.get_raw()+"|.", "g");
        },
        clear_not_matching: function (v){
          const pattern = this.build();
          let parts = null;
          let result = '';
          while(parts = pattern.exec(v)){
            result += parts[0];
          }
          return result;
        }
    }
};


// Aqu'i empieza.
let pb = make_pattern_builder();

pb.add_chars("a-zA-Z ");
pb.add_autoclosing_tag("br");

console.log(pb.clear_not_matching("hola <br> / 6666 que tal"));