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Regex for a valid hashtag

I need regular expression for validating a hashtag. Each hashtag should starts with hashtag(“#”).

Valid inputs:

1. #hashtag_abc

2. #simpleHashtag

3. #hashtag123

Invalid inputs:

1. #hashtag#

2. #[email protected]

I have been trying with this regex /#[a-zA-z0-9]/ but it is accepting invalid inputs also.

Any suggestions for how to do it?

Answer

To answer the current question…

There are 2 issues:

Since you are validating the whole string, you also need anchors (^ and $)to ensure a full string match:

/^#w+$/

See the regex demo.

If you want to extract specific valid hashtags from longer texts…

This is a bonus section as a lot of people seek to extract (not validate) hashtags, so here are a couple of solutions for you. Just mind that w in JavaScript (and a lot of other regex libraries) equal to [a-zA-Z0-9_]:

  • #w{1,30}b – a # char followed with one to thirty word chars followed with a word boundary
  • B#w{1,30}b – a # char that is either at the start of string or right after a non-word char, then one to thirty word (i.e. letter, digit, or underscore) chars followed with one to thirty word chars followed with a word boundary
  • B#(?![d_]+b)(w{1,30})b# that is either at the start of string or right after a non-word char, then one to thirty word (i.e. letter, digit, or underscore) chars (that cannot be just digits/underscores) followed with a word boundary

And last but not least, here is a Twitter hashtag regex from https://github.com/twitter/twitter-text/tree/master/js... Sorry, too long to paste in the SO post, here it is: https://gist.github.com/stribizhev/715ee1ee2dc1439ffd464d81d22f80d1.