const regex = /(?:<ins class="thisisfirstscript")([\s\S]*?)(?:data-slot="4397140188"><\/ins>)/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?:<ins class="thisisfirstscript")([\\s\\S]*?)(?:data-slot="4397140188"><\\\/ins>)', 'gm')
const str = `<ins class="thisisfirstscript"
style="display:inline-block;width:336px;height:280px"
data-client="ABCD"
data-slot="4397140188"></ins>
<ins class="thisissecondscript"
style="display:inline-block;width:336px;height:280px"
data-client="CDEF"
data-slot="4496122889"></ins>
<ins class="thisisfirstscript"
style="display:inline-block;width:336px;height:280px"
data-client="ABCD"
data-slot="4397140188"></ins>`;
const subst = `$1index.php`;
// 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