/
(?<First>\w+?) (?# First word UNGREEDY)
(?> (?# Second group - see it is optional)
(?>_) (?# But if it exists, must start with "_" NON CAPTURING GROUP )
(?<Second>\w+) (?# Snd then there should be a word - at least one char)
)* (?# remember - the group is optional)
\b (?# All the puttern must finish with word boundary)
/
gmx