const regex = /(?=[A-Z][a-z])(?<!^)|(?=[A-Z])(?<=[a-z])/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(?=[A-Z][a-z])(?<!^)|(?=[A-Z])(?<=[a-z])', 'gm')
const str = `FileName
ISO
InternalNDFilter
ThisIsAStringOfWords
`;
const subst = ` `;
// 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