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hashtags are the use of the hash "#" symbol followed immediately by a word/phrase/abbreviation to explicitly denote a topic inline in a post. Popularized on Twitter, they're now everpresent on Instagram and other silos where they link to aggregations of posts inside those silos with those tags.

This page is specifically about hashtags. For general concept of tags and tagging content, see:


Use rel=tag on hashtag links. If you're using h-entry, use p-category as well.


How do we discover indieweb posts tagged with a particular tag? It’s a problem well suited to centralised hubs.

Could we get services like tagboard to start indexing indieweb posts, not just posts in silos?

IndieWeb examples

Aaron Parecki

Aaron Parecki uses p3k on aaronparecki.com to auto-link hashtags on his posts to his own tag aggregation page since 2013-03-08.

Barnaby Walters

Barnaby Walters uses Taproot on waterpigs.co.uk to auto-link to a /tags page containing a list of links to content which is tagged with that tag (as per hypermedia API design, example) since 2012-09-22. This probably needs some js/iframe goodness to pull that content in --Waterpigs.co.uk 05:57, 19 April 2013 (PDT)


Main article: tags#Tantek

Tantek Çelik has published automatically marked up hashtags since 2016-01-26.

Jamie Tanna

  Jamie Tanna uses hashtags in his posts to tags on the post itself, so it can be used to categorise his content more easily. This has the added benefit of owning Twitter-specific interactions if using a certain hashtag. Since 2020-01-13, see a recent example with auto-linked hashtags


Autolinking hashtags

Where should hashtags in posts be auto-linked to?

Link them to a page on your site which aggregates posts tagged with that tag. It’s fine for this to just redirect to, for example, a google search of your site for that tag, but by always linking to a URL you control you can improve it/self-host it later.

Silo support

Silos that support hashtag aggregation on their own content.

Single post-type hashtag aggregations:

  • Twitter - notes (with embedded media, but not displayed by default, you have to twiddle them open)
  • Flickr - photos (and videos, but they don't play on tag aggregation pages, so effectively they're just images as well - poster-images from videos)

Composite post-type hashtag aggregations:

  • Tumblr - Tumbler has an amazing display of hetergenous post types in a single tag aggregation grid.


It appears that most platforms/silos limit hashtags to:

  • _a-zA-Z0-9 characters
  • Twitter, max length 139 (since Tweet length is 140)
    • Twitter max promoted trend (hashtag) is 20.[1]
  • most sites disallow hashtags that are just numbers (exception: Instagram allows them)
  • some sites disallow hashtags that start with numbers (exception: Twitter near the end of 2009 started supporting hashtags that start with numbers per the increased frequency of use, e.g. #2009resolution.)

See also:


person tag in plain text


Check out Twitter's regex: https://github.com/twitter/twitter-text/blob/master/js/twitter-text.js#L135-L141

The following are a coarse approximations which will miss numerous international (non-English) hashtags.

  • /#[_a-zA-Z0-9]{1,139}/ is a regex that will match common hashtags per the #Limitations documented above, while liberally allowing numbers-only and numbers-starting hashtags, limited to hashtags that will fit on Twitter.
  • /#\w{1,139}/ is shorter.

more characters

Tantek Çelik I have use-cases for more characters than the "usual" / Twitter’s regex. E.g. (from https://github.com/w3c/csswg-drafts/labels?page=1&sort=name-asc)

  • ' ': "Needs Thought" (will use "__" for ' ' per Falcon#tags_with_spaces as noted below)
  • '-': "css-ui"
  • '/': "unknown/future spec"

hashtags for tags with spaces


So having a way for hashtags to link to categories with spaces is desirable

IndieWeb-centric hashtags

Some hashtags commonly used by the IndieWeb community:

  • #IndieWeb (naturally)
  • #IndieWebCamp
  • #newwwyear
    • Started by Jen Simmons on Twitter as a means of using the idea of New Year's resolutions for taking small, but concrete steps towards building and improving one's website

See Also