const regex = /(( *?)--([A-z\-0-9]*?): (#[a-z0-9]*?);)/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('(( *?)--([A-z\\-0-9]*?): (#[a-z0-9]*?);)', 'gm')
const str = ` --color-primary: #0017a8;
--variablename: blabla blablabla;
--color-secondary: #0631ff;
--color-tertiary: #00ffe0;
--color-green-dark: #257426;
`;
const subst = `$1\n$2--$3--rgb: #{$hexToRGB($3)};`;
// 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