matching a line that doesn't contain specific text with regular expressions

Question

I want to do the following with regular expressions but not sure how to do it. I want it to match one two when one two is the beginning of the line

Answer 1

^one two(?!.*three)

Answer 2

You need a negative lookahead assertion - something like this:

/^one two(?!.*three)/m

Here's a tutorial on lookahead/lookbehind assertions

Note: I've added the 'm' modifier so that ^ matches the start of a line rather than the start of the whole string.