const regex = /(<FO_KEP>[^<]*<\/FO_KEP>)\r?\n(<OSSZESZERELESI_KEP>[^<]*<\/OSSZESZERELESI_KEP>)/g;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(<FO_KEP>[^<]*<\\\/FO_KEP>)\\r?\\n(<OSSZESZERELESI_KEP>[^<]*<\\\/OSSZESZERELESI_KEP>)', 'g')
const str = `<TERMEK>
<TERMEK_KOD>01008213</TERMEK_KOD>
<FO_KEP>https://www.temponabytok.sk/Files/obrazky/01008213.jpg</FO_KEP>
<OSSZESZERELESI_KEP>https://www.temponabytok.sk/Files/dokumenty/Vratko_montazny_navod.jpg</OSSZESZERELESI_KEP>
<OBRAZKY>https://www.temponabytok.sk/Files/2341000/2341428/245320712.jpg</OBRAZKY>
</TERMEK>`;
const subst = `\2\n\1`;
// 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