const regex = /\s*on\s*(?!.*on)/gim;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('\\s*on\\s*(?!.*on)', 'gim')
const str = `Syed on Arafath on Jan 7, 2015
Capt.KSD on Dec 30, 2014
chakradharalasakani on Dec 29, 2014
mitesh0123 on Dec 18, 2014
Aparajita61@yahoo.in on Dec 3, 2014
123chetan on Oct 28, 2014`;
const subst = `\n`;
// 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