const regex = /^((?:.*\n){5})((?:.*\n){5})/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('^((?:.*\\n){5})((?:.*\\n){5})', 'gm')
const str = `1
2
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5
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`;
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