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

This should work for you

\b[dog]+\b(?![,])

Explanation

r"""
\b        # Assert position at a word boundary
[dog]     # Match a single character present in the list “dog”
   +         # Between one and unlimited times, as many times as possible, giving back as needed (greedy)
\b        # Assert position at a word boundary
(?!       # Assert that it is impossible to match the regex below starting at this position (negative lookahead)
   [,]       # Match the character “,”
)
"""