const regex = new RegExp('(?:(//\\w+\\s+JOB(?: \\w+)?)\\h*|\\G(?!^)),(\\w+=\\w+)', 'gm')
const str = `//ADBB503 JOB ,MSGCLASS=2,CLASS=P
//ABCD JOB Something,MSG=NTNG,CLASS=ABC`;
const subst = ``;
// The substituted value will be contained in the result variable
const result = str.replace(regex, subst);
console.log('Substitution result: ', result);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for JavaScript, please visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Guide/Regular_Expressions