REGEXP and expressions beginning at start of string or with a separator

I'd like a REGEXP that only picks up a substring that either starts at the beginning开发者_JS百科 of the string or is preceded by a space or separating character. How does this work?

For example,

[ _^]Smith

seems to be almost it!

(^|[ _])Smith

^ (start) or one of [ _] followed by Smith

(| is the or operator)

Something like this:

^[ _]?Smith

^- beginning of line
[ _]? - space or underscore 0 or one time