const regex = /(?m)(.*\n)*(.+?)(5)\[(.*)\](.\n)*(.*\n*)*/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?m)(.*\\n)*(.+?)(5)\\[(.*)\\](.\\n)*(.*\\n*)*', 'gm')
const str = `SHIELD1[1SHIELD,2SHIELDCAP,3SHIELDPERCENT,]
ENERGY2[1POWER,2POWERCAP,3POWERPERCENT,]
STRUCTURE3[1STRUCTUREHP,2STRUCTUREHPCAP,3STRUCTUREHPPERCENT,]
ARMOR4[1ARMORHP,2ARMORHPCAP,3ARMORHPPERCENT,]
OTHER5[1MASS,2BLOCKCOUNT,3SECTOR,4SYSTEM,]
SELECT THE MENU YOU WANT
[REPLACEFIRST](.*\\n)*(.+?)(3)\\[(.*)\\](.\\n)*(.*\\n*)*[WITH]\$4`;
const subst = `$4`;
// 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