[рони]

sovsem-nub,

let str="null++test alert system1_20191109110452333++test alert system_20191109110452444++test ии аа_20191109110452555";
let txt = "++test alert system1";
let test = "null++test alert system_20191109110452444++test ии аа_20191109110452555"
const escapeRegExp = string => string.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
const fn = (str, txt) =>{
    const reg = new RegExp( escapeRegExp(txt) + ".{18}", "g");
    return str.replace(reg, "")
}
alert(test == fn(str, txt));