const regex = new RegExp('<a\\s[^>]*href="https?://[^>]*>(.*?)</a>', 'gis')
const str = `<a href="https://site.ru/*****>слово</a>
<a class="m" href="https://site.ru/*****>слово</a>
<a href="https://site.ru/*****" class="m" >слово</a>
<a href="https://www.site.ru/sistemy-promyshlennogo-konditsionirovaniya/fankoyly/ballu-machine/" rel="nofollow">слово</a> <br>
<a href="https://www.site2.ru/fdescript/1559/?design=1">слово</a> <br>
<a href="https://www.site3.ru/fdescript/997/?design=1">слово</a> <br>
<a href="https://www.site3.ru/fdescript/1003/?design=1" rel="nofollow">слово</a>
`;
const subst = `\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