Question

I am trying to extract hashtags for a simple college project using ruby on rails. I am facing issue with tags that include only numericals and with tags with no space.

text = "Pack my #box with #5 dozen liquor.#jugs link.com/liquor#jugs #2good #first#second"

The regex I have is /(?:^|\s)#(\w+)/i (source)

This regex returns #["box", "5", "2good", "first"]

How to make sure it only returns #["box", "2good"] and ignore the rest as they are not ‘real’ hashtags?

Answer 1

Can you try this regex:

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

UPDATE 1:
There are a few cases where the above regex will not match like: #blah23blah and #23blah23.
Hence modified the regex to take care of all cases.

Regex:

/(?:\s|^)(?:#(?!\d+(?:\s|$)))(\w+)(?=\s|$)/i

Breakdown:

• (?:\s|^) –Matches the preceding space or start of line. Does not
  capture the match.
• # –Matches hash but does not capture.
• (?!\d+(?:\s|$))) –Negative Lookahead to avoid ALL numeric characters
  between # and space (or end of line)
• (\w+) –Matches and captures all word characters
• (?=\s|$) –Positive Lookahead to ensure following space or end of
  line. This is required to ensure it matches adjacent valid hash tags.

Sample text modified to capture most cases:

#blah Pack my #box with #5 dozen #good2 #3good liquor.#jugs
link.com/liquor#jugs #mkvef214asdwq sd #3e4 flsd #2good #first#second #3

Matches:

Match 1: blah
Match 2: box
Match 3: good2
Match 4: 3good
Match 5: mkvef214asdwq
Match 6: 3e4
Match 7: 2good

Rubular link

UPDATE 2:

To exclude words starting or ending with underscore, just include your exclusions in the negative lookahead like this:

/(?:\s|^)(?:#(?!(?:\d+|\w+?_|_\w+?)(?:\s|$)))(\w+)(?=\s|$)/i

The sample, regex and matches are recorded in this Rubular link