Regular expression to get a string between two strings where the last string can also be 'end of string'
URLSearchParams would make things significantly easier:


const query1 = '?someBoolean=false&amp;q=&amp;location=&amp;testParam=dummy_value&amp;testParam2=dummy_value2&amp;requiredParam=requiredValue';
const query2 = '?someBoolean=false&amp;q=&amp;location=&amp;testParam=dummy_value&amp;testParam2=dummy_value2&amp;requiredParam=requiredValue&amp;someMoreParam=dummy_value2';

const params1 = new URLSearchParams(query1);
const params2 = new URLSearchParams(query2);

console.log(`For Query1: result = ${params1.get('requiredParam')}`);
console.log(`For Query2: result = ${params2.get('requiredParam')}`);



It's supported natively in the vast majority of browsers, but not all. For the rest, here's a polyfill. It's better not to re-invent the wheel when you don't need to, and it's good when you're able to use a standard API (with examples and documentation and Stack Overflow answers about it, etc).
As a side note - when using regular expressions, I'd recommend using capture groups only when necessary. If all you need to do is group some tokens together logically (like for a | alternation), non-capturing groups should be preferred. That is, if URLSearchParams didn't exist, better to do (?:&amp;|$) than (&amp;|$). Reserve capturing groups for when you need to save and use the captured result somewhere - otherwise, non-capturing groups are more appropriate, less expensive, and require less cognitive overhead.
If you had to go the regex route, another slight improvement would be to use a negative character class instead of lazy repetition. In the pattern, you have:
(.*?)(&amp;|$)

Lazy repetition is slow; it forces the engine to advance one character at a time, then check the rest of the pattern for a match, and repeat until the match is found. Since you know that the capture group will not contain any &amp;s, better to match anything but &amp;s:
([^&amp;]*)

Once you do that, you don't even need the final (&amp;|$) or (?:&amp;|$) due to the greedy repetition.