You want a negative lookahead or a negative lookbehind. Suppose you want to validate so that a phrase starts with "string" and ends with "bee", but does not contain "like". You could use this regular expression (which uses a negative lookahead):
The negative look ahead will ensure that the expression will not match if the phrase contains "like". For example, "sting as if a bee" would be a match, but "sting like a bee" would not match (because it contains "like"). Now, if you want to disallow both "like" and "as if", the modification is simple:
Now neither of the sample phrases I gave above will match.