const regex = /\!\[.+?\]\((.+?)\)/gm;
// Alternative syntax using RegExp constructor
// const regex = new RegExp('\\!\\[.+?\\]\\((.+?)\\)', 'gm')
const str = `![Screenshot_20210618-090636](/uploads/7afe1f092f836d61a5fa008cf9a0c991/Screenshot_20210618-090636.jpg)
![screenshot](uploads/9d461c7a3fd8cffb4c7a84c1948f345f/bootloader.jpg)`;
const subst = `<img src="$1" width="300">`;
// 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