regex to match entire words containing only certain characters

Question

I want to match entire words (or strings really) that containing only defined characters.

For example if the letters are d, o, g

Answer 1

The following regex represents one or more occurrences of the three characters you're looking for:

[dog]+

Explanation:

The square brackets mean: "any of the enclosed characters".

The plus sign means: "one or more occurrences of the previous expression"

This would be the exact same thing:

[ogd]+