Regex for a valid hashtag

Question

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. #hashtag@hashtag

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 1

The current accepted answer fails in a few places:

• It accepts hashtags that have no letters in them (i.e. "#11111", "#___" both pass).
• It will exclude hashtags that are separated by spaces ("hey there #friend" fails to match "#friend").
• It doesn't allow you to place a min/max length on the hashtag.
• It doesn't offer a lot of flexibility if you decide to add other symbols/characters to your valid input list.

Try the following regex:

/(^|\B)#(?![0-9_]+\b)([a-zA-Z0-9_]{1,30})(\b|\r)/g

It'll close up the above edge cases, and furthermore:

• You can change {1,30} to your desired min/max
• You can add other symbols to the [0-9_] and [a-zA-Z0-9_] blocks if you wish to later

Here's a link to the demo.

Answer 2

To answer the current question...

There are 2 issues:

• [A-z] allows more than just letter chars ([, , ], ^, _, ` )
• There is no quantifier after the character class and it only matches 1 char

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.