an rgb() colour value

Matches an rgb() colour value. It is anchored at both ends, so the whole string has to match. There are no capture groups.

Pattern

/^rgb$\s*\d{1,3}\s*,\s*\d{1,3}\s*,\s*\d{1,3}\s*$$/

Try it yourself

Type any text and the parts this expression catches are highlighted. It runs in your browser and is never sent anywhere.

rgb(255, 0, 0)

1 match

With ^ and $ around it, the whole string must match; without them the match can sit anywhere. Always anchor an expression you use for validation.
Round brackets ( ) capture what they wrap. When you only need grouping, write (?: ) so the numbering of later groups does not shift.
Every expression here is checked against its examples. Even so, "looks like an email" and "is a real inbox" remain different questions.
Nested open-ended repeats such as (a+)+ can blow up on certain inputs. Look for that shape before pasting someone else’s expression into your code.

QWhat is the regex for an rgb() colour value?

^rgb$\s*\d{1,3}\s*,\s*\d{1,3}\s*,\s*\d{1,3}\s*$$.

QWhich strings match?

Text like rgb(255, 0, 0) matches; text like rgb(255 0 0) does not.

QDoes the whole string have to match?

Yes. It is wrapped in ^ and $, so nothing but a full match passes.

QHow do I use what it caught?

There are no capture groups, so only the whole match (match[0]) comes back. Wrap the part you need in brackets to capture it.