Which characters are allowed in hashtags

Question

I am having a really hard time figuring out a regular expression (in C#) to validate hashtags. \w simply isn't enough as special characters are missing (ä, ö, ø, æ, å for starters, but also a lot of other foreign characters.

I need to support ALL hashtags there is. Mainly from Twitter, but in the future also from other providers.

My best shot (so far) is: ^#[a-zA-Z_0-9\u00C0-\u02AF]+$ (C# regex)

I cannot find any decent documentation from Twitter or anyone else about this, so:

• Does anyone know of any documentation I have missed?
• OR does anyone know which unicode ranges I should include as valid characters for hashtags?
• AND Can anybody tell me if there is a difference between the support of hashtags on e.g. Twitter, Instagram, Facebook, etc.?

Update I should note that C# is not the only language I need this in. Thus the need for precise specification.

Answer 1

A Quick-and-Dirty Simplified Approach

Here is a nice-read from Twitter eng team:

To be fair, the Twitter team do have a standard. Even if they don't use it themselves.

The test cases and other valuable information is located at https://github.com/twitter/twitter-text/blob/master/java/src/test/java/com/twitter/twittertext/RegexTest.java. Acc. to it, the valid hashtag can be written in C# as

(^|\s)([#＃][\w\u05be\u05f3\u05f4]*[\p{L}_]+[\w\u05be\u05f3\u05f4]*)

See this regex demo

Since you want to be able to use this in any language, just note that \p{L} is equal to

[A-Za-z\xAA\xB5\xBA\xC0-\xD6\xD8-\xF6\xF8-\u02C1\u02C6-\u02D1\u02E0-\u02E4\u02EC\u02EE\u0370-\u0374\u0376\u0377\u037A-\u037D\u037F\u0386\u0388-\u038A\u038C\u038E-\u03A1\u03A3-\u03F5\u03F7-\u0481\u048A-\u052F\u0531-\u0556\u0559\u0561-\u0587\u05D0-\u05EA\u05F0-\u05F2\u0620-\u064A\u066E\u066F\u0671-\u06D3\u06D5\u06E5\u06E6\u06EE\u06EF\u06FA-\u06FC\u06FF\u0710\u0712-\u072F\u074D-\u07A5\u07B1\u07CA-\u07EA\u07F4\u07F5\u07FA\u0800-\u0815\u081A\u0824\u0828\u0840-\u0858\u08A0-\u08B4\u0904-\u0939\u093D\u0950\u0958-\u0961\u0971-\u0980\u0985-\u098C\u098F\u0990\u0993-\u09A8\u09AA-\u09B0\u09B2\u09B6-\u09B9\u09BD\u09CE\u09DC\u09DD\u09DF-\u09E1\u09F0\u09F1\u0A05-\u0A0A\u0A0F\u0A10\u0A13-\u0A28\u0A2A-\u0A30\u0A32\u0A33\u0A35\u0A36\u0A38\u0A39\u0A59-\u0A5C\u0A5E\u0A72-\u0A74\u0A85-\u0A8D\u0A8F-\u0A91\u0A93-\u0AA8\u0AAA-\u0AB0\u0AB2\u0AB3\u0AB5-\u0AB9\u0ABD\u0AD0\u0AE0\u0AE1\u0AF9\u0B05-\u0B0C\u0B0F\u0B10\u0B13-\u0B28\u0B2A-\u0B30\u0B32\u0B33\u0B35-\u0B39\u0B3D\u0B5C\u0B5D\u0B5F-\u0B61\u0B71\u0B83\u0B85-\u0B8A\u0B8E-\u0B90\u0B92-\u0B95\u0B99\u0B9A\u0B9C\u0B9E\u0B9F\u0BA3\u0BA4\u0BA8-\u0BAA\u0BAE-\u0BB9\u0BD0\u0C05-\u0C0C\u0C0E-\u0C10\u0C12-\u0C28\u0C2A-\u0C39\u0C3D\u0C58-\u0C5A\u0C60\u0C61\u0C85-\u0C8C\u0C8E-\u0C90\u0C92-\u0CA8\u0CAA-\u0CB3\u0CB5-\u0CB9\u0CBD\u0CDE\u0CE0\u0CE1\u0CF1\u0CF2\u0D05-\u0D0C\u0D0E-\u0D10\u0D12-\u0D3A\u0D3D\u0D4E\u0D5F-\u0D61\u0D7A-\u0D7F\u0D85-\u0D96\u0D9A-\u0DB1\u0DB3-\u0DBB\u0DBD\u0DC0-\u0DC6\u0E01-\u0E30\u0E32\u0E33\u0E40-\u0E46\u0E81\u0E82\u0E84\u0E87\u0E88\u0E8A\u0E8D\u0E94-\u0E97\u0E99-\u0E9F\u0EA1-\u0EA3\u0EA5\u0EA7\u0EAA\u0EAB\u0EAD-\u0EB0\u0EB2\u0EB3\u0EBD\u0EC0-\u0EC4\u0EC6\u0EDC-\u0EDF\u0F00\u0F40-\u0F47\u0F49-\u0F6C\u0F88-\u0F8C\u1000-\u102A\u103F\u1050-\u1055\u105A-\u105D\u1061\u1065\u1066\u106E-\u1070\u1075-\u1081\u108E\u10A0-\u10C5\u10C7\u10CD\u10D0-\u10FA\u10FC-\u1248\u124A-\u124D\u1250-\u1256\u1258\u125A-\u125D\u1260-\u1288\u128A-\u128D\u1290-\u12B0\u12B2-\u12B5\u12B8-\u12BE\u12C0\u12C2-\u12C5\u12C8-\u12D6\u12D8-\u1310\u1312-\u1315\u1318-\u135A\u1380-\u138F\u13A0-\u13F5\u13F8-\u13FD\u1401-\u166C\u166F-\u167F\u1681-\u169A\u16A0-\u16EA\u16F1-\u16F8\u1700-\u170C\u170E-\u1711\u1720-\u1731\u1740-\u1751\u1760-\u176C\u176E-\u1770\u1780-\u17B3\u17D7\u17DC\u1820-\u1877\u1880-\u18A8\u18AA\u18B0-\u18F5\u1900-\u191E\u1950-\u196D\u1970-\u1974\u1980-\u19AB\u19B0-\u19C9\u1A00-\u1A16\u1A20-\u1A54\u1AA7\u1B05-\u1B33\u1B45-\u1B4B\u1B83-\u1BA0\u1BAE\u1BAF\u1BBA-\u1BE5\u1C00-\u1C23\u1C4D-\u1C4F\u1C5A-\u1C7D\u1CE9-\u1CEC\u1CEE-\u1CF1\u1CF5\u1CF6\u1D00-\u1DBF\u1E00-\u1F15\u1F18-\u1F1D\u1F20-\u1F45\u1F48-\u1F4D\u1F50-\u1F57\u1F59\u1F5B\u1F5D\u1F5F-\u1F7D\u1F80-\u1FB4\u1FB6-\u1FBC\u1FBE\u1FC2-\u1FC4\u1FC6-\u1FCC\u1FD0-\u1FD3\u1FD6-\u1FDB\u1FE0-\u1FEC\u1FF2-\u1FF4\u1FF6-\u1FFC\u2071\u207F\u2090-\u209C\u2102\u2107\u210A-\u2113\u2115\u2119-\u211D\u2124\u2126\u2128\u212A-\u212D\u212F-\u2139\u213C-\u213F\u2145-\u2149\u214E\u2183\u2184\u2C00-\u2C2E\u2C30-\u2C5E\u2C60-\u2CE4\u2CEB-\u2CEE\u2CF2\u2CF3\u2D00-\u2D25\u2D27\u2D2D\u2D30-\u2D67\u2D6F\u2D80-\u2D96\u2DA0-\u2DA6\u2DA8-\u2DAE\u2DB0-\u2DB6\u2DB8-\u2DBE\u2DC0-\u2DC6\u2DC8-\u2DCE\u2DD0-\u2DD6\u2DD8-\u2DDE\u2E2F\u3005\u3006\u3031-\u3035\u303B\u303C\u3041-\u3096\u309D-\u309F\u30A1-\u30FA\u30FC-\u30FF\u3105-\u312D\u3131-\u318E\u31A0-\u31BA\u31F0-\u31FF\u3400-\u4DB5\u4E00-\u9FD5\uA000-\uA48C\uA4D0-\uA4FD\uA500-\uA60C\uA610-\uA61F\uA62A\uA62B\uA640-\uA66E\uA67F-\uA69D\uA6A0-\uA6E5\uA717-\uA71F\uA722-\uA788\uA78B-\uA7AD\uA7B0-\uA7B7\uA7F7-\uA801\uA803-\uA805\uA807-\uA80A\uA80C-\uA822\uA840-\uA873\uA882-\uA8B3\uA8F2-\uA8F7\uA8FB\uA8FD\uA90A-\uA925\uA930-\uA946\uA960-\uA97C\uA984-\uA9B2\uA9CF\uA9E0-\uA9E4\uA9E6-\uA9EF\uA9FA-\uA9FE\uAA00-\uAA28\uAA40-\uAA42\uAA44-\uAA4B\uAA60-\uAA76\uAA7A\uAA7E-\uAAAF\uAAB1\uAAB5\uAAB6\uAAB9-\uAABD\uAAC0\uAAC2\uAADB-\uAADD\uAAE0-\uAAEA\uAAF2-\uAAF4\uAB01-\uAB06\uAB09-\uAB0E\uAB11-\uAB16\uAB20-\uAB26\uAB28-\uAB2E\uAB30-\uAB5A\uAB5C-\uAB65\uAB70-\uABE2\uAC00-\uD7A3\uD7B0-\uD7C6\uD7CB-\uD7FB\uF900-\uFA6D\uFA70-\uFAD9\uFB00-\uFB06\uFB13-\uFB17\uFB1D\uFB1F-\uFB28\uFB2A-\uFB36\uFB38-\uFB3C\uFB3E\uFB40\uFB41\uFB43\uFB44\uFB46-\uFBB1\uFBD3-\uFD3D\uFD50-\uFD8F\uFD92-\uFDC7\uFDF0-\uFDFB\uFE70-\uFE74\uFE76-\uFEFC\uFF21-\uFF3A\uFF41-\uFF5A\uFF66-\uFFBE\uFFC2-\uFFC7\uFFCA-\uFFCF\uFFD2-\uFFD7\uFFDA-\uFFDC]

and \w is a combination of \p{L}, _ and a \p{N}, see \p{N} below:

[0-9\xB2\xB3\xB9\xBC-\xBE\u0660-\u0669\u06F0-\u06F9\u07C0-\u07C9\u0966-\u096F\u09E6-\u09EF\u09F4-\u09F9\u0A66-\u0A6F\u0AE6-\u0AEF\u0B66-\u0B6F\u0B72-\u0B77\u0BE6-\u0BF2\u0C66-\u0C6F\u0C78-\u0C7E\u0CE6-\u0CEF\u0D66-\u0D75\u0DE6-\u0DEF\u0E50-\u0E59\u0ED0-\u0ED9\u0F20-\u0F33\u1040-\u1049\u1090-\u1099\u1369-\u137C\u16EE-\u16F0\u17E0-\u17E9\u17F0-\u17F9\u1810-\u1819\u1946-\u194F\u19D0-\u19DA\u1A80-\u1A89\u1A90-\u1A99\u1B50-\u1B59\u1BB0-\u1BB9\u1C40-\u1C49\u1C50-\u1C59\u2070\u2074-\u2079\u2080-\u2089\u2150-\u2182\u2185-\u2189\u2460-\u249B\u24EA-\u24FF\u2776-\u2793\u2CFD\u3007\u3021-\u3029\u3038-\u303A\u3192-\u3195\u3220-\u3229\u3248-\u324F\u3251-\u325F\u3280-\u3289\u32B1-\u32BF\uA620-\uA629\uA6E6-\uA6EF\uA830-\uA835\uA8D0-\uA8D9\uA900-\uA909\uA9D0-\uA9D9\uA9F0-\uA9F9\uAA50-\uAA59\uABF0-\uABF9\uFF10-\uFF19]

and whitespace is something like

[\t\f\v\r\n\x20\xA0\u1680\u2000-\u200A\u2028\u2029\u202F\u205F\u3000]

Note there can be issues with diacritic matching in ES5 regex syntax.

UPDATE

twitter-text C# Adaptation

The Java library features the following regex for the hashtags:

VALID_HASHTAG = Pattern.compile("(^|[^&" + HASHTAG_LETTERS_NUMERALS + "])(#|\uFF03)(?!\uFE0F|\u20E3)(" + HASHTAG_LETTERS_NUMERALS_SET + "*" + HASHTAG_LETTERS_SET + HASHTAG_LETTERS_NUMERALS_SET + "*)", Pattern.CASE_INSENSITIVE);

Translating into C#:

string HASHTAG_LETTERS = @"\p{L}\p{M}";
string HASHTAG_NUMERALS = @"\p{Nd}";
string HASHTAG_SPECIAL_CHARS = @"_\u200c\u200d\ua67e\u05be\u05f3\u05f4\uff5e\u301c\u309b\u309c\u30a0\u30fb\u3003\u0f0b\u0f0c\u00b7";
string HASHTAG_LETTERS_NUMERALS = HASHTAG_LETTERS + HASHTAG_NUMERALS + HASHTAG_SPECIAL_CHARS;
string HASHTAG_LETTERS_NUMERALS_SET = "[" + HASHTAG_LETTERS_NUMERALS + "]";
string HASHTAG_LETTERS_SET = "[" + HASHTAG_LETTERS + "]";
string VALID_HASHTAG = new Regex("(^|[^&" + HASHTAG_LETTERS_NUMERALS + @"])(#|\uFF03)(?!\uFE0F|\u20E3)(" + HASHTAG_LETTERS_NUMERALS_SET + "*" + HASHTAG_LETTERS_SET + HASHTAG_LETTERS_NUMERALS_SET + "*)", RegexOptions.IgnoreCase);

And here is a testing C# demo:

using System;
using System.Linq;
using System.Collections.Generic;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {
        string HASHTAG_LETTERS = @"\p{L}\p{M}";
        string HASHTAG_NUMERALS = @"\p{Nd}";
        string HASHTAG_SPECIAL_CHARS = @"_\u200c\u200d\ua67e\u05be\u05f3\u05f4\uff5e\u301c\u309b\u309c\u30a0\u30fb\u3003\u0f0b\u0f0c\u00b7";
        string HASHTAG_LETTERS_NUMERALS = HASHTAG_LETTERS + HASHTAG_NUMERALS + HASHTAG_SPECIAL_CHARS;
        string HASHTAG_LETTERS_NUMERALS_SET = "[" + HASHTAG_LETTERS_NUMERALS + "]";
        string HASHTAG_LETTERS_SET = "[" + HASHTAG_LETTERS + "]";
        Regex VALID_HASHTAG = new Regex("(^|[^&" + HASHTAG_LETTERS_NUMERALS + @"])(#|\uFF03)(?!\uFE0F|\u20E3)(" + HASHTAG_LETTERS_NUMERALS_SET + "*" + HASHTAG_LETTERS_SET + HASHTAG_LETTERS_NUMERALS_SET + "*)", RegexOptions.IgnoreCase);
        
        List<string> tests = new List<string>() {"#hashtag",
            "#Azərbaycanca",
            "#mûǁae",
            "#Čeština",
            "#Ċaoiṁín",
            "#Caoiṁín",
            "#táim",
            "#hag̃ua",
            "#café",
            "#עברית",
            "#אֲשֶׁר",
            "#עַל־יְדֵי",
            "#וכו׳",
            "#מ״כ",
            "#العربية",
            "#حالياً",
            "#يـﮱـَٱ",
            "#ประเทศไทย",
            "#ฟรี",
            "#日本語ハッシュタグ",
            "＃日本語ハッシュタグ",
            "これはOK #ハッシュタグ",
            "これもOK。#ハッシュタグ",
            "これはダメ#ハッシュタグ",
            "#1",
            "#2"};
        tests.ForEach(input => // JUST A PIECE OF DEMO CODE
            Console.WriteLine("Input: " + input + " = " + VALID_HASHTAG.IsMatch(input) +
              (VALID_HASHTAG.IsMatch(input) ? ", match = " + VALID_HASHTAG.Match(input).Value : "")));
    }
}

JavaScript Hashtag validation

If you use JS Twitter library, identifying hasgtags can be done with a mere:

var isValidHashtag = twttr.txt.isValidHashtag(hashtag);