| Unbound | Text.Trifecta.Highlight.Prim |
| Underline | Text.Trifecta |
| underline | Text.Trifecta |
| unexpected | Text.Trifecta.Parser.Class, Text.Trifecta.Parser, Text.Trifecta |
| unicodeCategories | Text.Trifecta.CharSet.Unicode |
| UnicodeCategory | |
| 1 (Type/Class) | Text.Trifecta.CharSet.Unicode |
| 2 (Data Constructor) | Text.Trifecta.CharSet.Unicode |
| unifiedCanadianAboriginalSyllabics | Text.Trifecta.CharSet.Unicode.Block |
| union | Text.Trifecta.CharSet |
| unlanguage | Text.Trifecta.Parser.Language.Monad, Text.Trifecta.Parser.Language, Text.Trifecta.Parser, Text.Trifecta |
| unlayout | Text.Trifecta.Parser.Layout.Monad, Text.Trifecta.Parser.Layout, Text.Trifecta.Parser, Text.Trifecta |
| unparser | Text.Trifecta.Parser.Prim, Text.Trifecta.Parser, Text.Trifecta |
| upper | |
| 1 (Function) | Text.Trifecta.CharSet.Common |
| 2 (Function) | Text.Trifecta.CharSet.Posix.Ascii |
| 3 (Function) | Text.Trifecta.CharSet.Posix.Unicode |
| 4 (Function) | Text.Trifecta.Parser.Char, Text.Trifecta.Parser, Text.Trifecta |
| 5 (Function) | Text.Trifecta.Parser.Char8 |
| uppercaseLetter | |
| 1 (Function) | Text.Trifecta.CharSet.Unicode |
| 2 (Function) | Text.Trifecta.CharSet.Unicode.Category |