const regex = /<img src=([a-zA-Z0-9_\-\s\.\"=\/:%,()]*class=\"[a-zA-Z0-9_\-\s\.\"=\/:%,()]*lazy[a-zA-Z0-9_\-\s\.\"=\/:%,()]*)/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('<img src=([a-zA-Z0-9_\\-\\s\\.\\"=\\\/:%,()]*class=\\"[a-zA-Z0-9_\\-\\s\\.\\"=\\\/:%,()]*lazy[a-zA-Z0-9_\\-\\s\\.\\"=\\\/:%,()]*)', 'gm')
const str = ` <img src="fonts/608be7747d9cf15397b0d112_Logo-v3-01.svg" alt class="nav-logo">
<img src="images/6085e0acf284d920e7133043_Frame%201%20%283%29.png" loading="lazy" width="500" style="opacity:0" srcset="images/6085e0acf284d920e7133043_Frame%201%20%283%29-p-500.png 500w, images/6085e0acf284d920e7133043_Frame%201%20%283%29.png 738w" sizes="(max-width: 479px) 73vw, (max-width: 767px) 77vw, (max-width: 991px) 273.991455078125px, 380px" alt class="image_header">
<img src="images/6089c5d4b424624dc0cb450b_Frame%206.png" loading="lazy" width="263" sizes="(max-width: 479px) 65vw, (max-width: 767px) 77vw, (max-width: 991px) 33vw, 37vw" srcset="images/6089c5d4b424624dc0cb450b_Frame%206-p-500.png 500w, images/6089c5d4b424624dc0cb450b_Frame%206.png 651w" alt class="image_performance lazy">
<img src="images/6089c5d52359073cf4b6cc80_Frame%205.png" loading="lazy" width="287" sizes="(max-width: 479px) 77vw, (max-width: 767px) 85vw, (max-width: 991px) 41vw, 43vw" srcset="images/6089c5d52359073cf4b6cc80_Frame%205-p-500.png 500w, images/6089c5d52359073cf4b6cc80_Frame%205.png 771w" alt class="image_security lazy">
<img src="images/6089c5d4e99fbfce103ab502_Frame%207.png" loading="lazy" width="292" sizes="(max-width: 479px) 71vw, (max-width: 767px) 81vw, (max-width: 991px) 263.991455078125px, 370px" srcset="images/6089c5d4e99fbfce103ab502_Frame%207-p-500.png 500w, images/6089c5d4e99fbfce103ab502_Frame%207.png 651w" alt class="image_reliability lazy">
<img src="fonts/608be7747d9cf15397b0d112_Logo-v3-01.svg" alt class="footer-logo-copy lazy">
<img src="fonts/60857020dc3fb163eb8a7d19_icons8-instagram.svg" loading="lazy" alt class="instagram_logo lazy">`;
const subst = `<img data-src=$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