an image file extension

Matches an image file extension. It is anchored at both ends, so the whole string has to match. There are no capture groups.

Pattern

Try it yourself

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

photo.jpg

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 image file extension?

QWhich strings match?

Text like photo.jpg matches; text like photo.txt 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.