const regex = new RegExp('(?<!^\\s*)(?=<GTOL-[A-Z]+>)', 'gm')
const str = `<GTOL-POSI>.010<MOD-MMC>B-C<GTOL-POSI>.002<MOD-FMC>
<GTOL-POSI><MOD-DIAM>.004<MOD-MMC>HC<MOD-MMC><GTOL-POSI>.030<MOD-MMC>D-E<MOD-FMC>
<GTOL-POSI><MOD-DIAM>.005<MOD-MMC>AD-E<MOD-MMC><GTOL-PERP>.001A`;
const subst = `\n\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