Regular expression to match a pattern either at the beginning of the line or after a space character

Question

I've been trying to dry up the following regexp that matches hashtags in a string with no success:

/^#(\w+)|\s#(\w+)/i

This won't work:

/^|\s#(\w+)/i

And no, I don't want to comma the alternation at the beginning:

/(^|\s)#(\w+)/i

I'm doing this in Ruby - though that should not be relevant I suppose.

To give some examples of matching and non-matching strings:

'#hashtag it is'        # should match => [["hashtag"]]
'this is a #hashtag'    # should match => [["hashtag"]]
'this is not a#hashtag' # should not match => []

Any suggestions? Am I nitpicking?

Answer 1

You can use.

/\B#(\w+)/i

"this is a #hash tag"      # matches
"#hash tag"                # matches
"this is not#hash tag"     # doesn't match

Answer 2

This uses look-behind and I don't know if look behinds are supported in Ruby (I heard that they are not supported in JavaScript)

/(^#(\w+))|((?<= )#(\w+))/

Answer 3

/(?:^|\s)#(\w+)/i

Adding the ?: prefix to the first group will cause it to not be a matching group, thus only the second group will actually be a matchgroup. Thus, each match of the string will have a single capturing group, the contents of which will be the hashtag.