Improve username regex

based on https://github.com/ornicar/lila/blob/master/modules/security/src/main/DataForm.scala#L34-L44

app/templating/StringHelper.scala

@@ -47,9 +47,12 @@ trait StringHelper { self: NumberHelper =>
  }
  }
 
- // Matches a lichess username with a '@' prefix (I hope), in the middle of word boundaries
+ // Matches a lichess username with a '@' prefix only if it doesn't follow another '@',
+ // if it isn't after a word character (that'd be an email) and fits constains in
+ // https://github.com/ornicar/lila/blob/master/modules/security/src/main/DataForm.scala#L34-L44
  // Example: everyone says @ornicar is a pretty cool guy
- private val atUsernameRegex = """\B(@([\w-]+))""".r
+ // False example: Write to [email protected], @1
+ private val atUsernameRegex = """\B@(?>\D[\w-]{1,19})(?![\w-])""".r
 
  private val urlRegex = """(?i)\b((?:https?:https://|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}/)(?:[^\s<>]+|\(([^\s<>]+|(\([^\s<>]+\)))*\))+(?:\(([^\s<>]+|(\([^\s<>]+\)))*\)|[^\s`!\[\]{};:'".,<>?«»“”‘’]))""".r