useval/highlight

val_highlight

Syntax highlighting for Dart and Flutter.

A pure-Dart engine with 55 languages, incremental updates and themes,
plus Flutter widgets to show and edit code.

val_highlight on pub.dev val_highlight_flutter on pub.dev BSD-3-Clause license

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📦 Packages

Package	Use it for
val_highlight	The engine, in pure Dart: 55 languages plus a generic fallback, flat tokens, incremental updates, language detection, 8 themes, VS Code theme import and HTML output. Runs on the VM, in Flutter and on the web.
val_highlight_flutter	Flutter widgets: CodeView to show code, CodeField to edit it, HighlightTextController, and CodeTheme for app-wide defaults.

A val_highlight_jaspr package is planned, built on the same engine.

// Flutter
CodeView(source, language: dartLanguage)

// Pure Dart
final html = const Highlighter().toHtml(source, language: dartLanguage);

Dart in CodeView with line numbers and highlighted lines CodeView Line numbers, highlighted lines, selection.	TSX in the midnight theme Themes Eight built in, or import a VS Code theme.
Markdown with bash, Python and JSON code fences Languages inside languages Markdown fences, HTML with CSS and JavaScript.	Python being edited in CodeField CodeField An editor that highlights as you type.

⚡ Performance

just bench compiles to native code, highlights a Dart file and renders it to HTML (best of 5 runs, on a MacBook):

Lines	Highlight + HTML
600	1.5 ms
2,400	4.3 ms
9,600	13.9 ms

just bench-languages measures about 256 KB of realistic source per language on native code. It averages about 43 MB/s, from 30 MB/s (HTML) to 64 MB/s (Markdown). just bench-js runs the same test compiled to JavaScript, where it averages about 62 MB/s.

With incremental highlighting, one keystroke in the 9,600-line file takes about 0.4 ms. Frame timings for scrolling and typing in Flutter are in the val_highlight_flutter README (just bench-frames).

How it works

• One pass, no copying. The scanner walks the source by offset, never copies it, and time grows in step with input size.
• First-character dispatch. Each rule's pattern is analysed once to find the characters it can start with. The scanner skips positions where no rule can start, and tries only the rules that can. Rules anchored with ^ are tried only at line starts. States with a rule that can start almost anywhere fall back to one combined regular expression. Both paths give identical results, and fuzz tests check this.
• Plain-Dart matchers. On native platforms, most patterns run as small Dart matchers instead of the regular expression engine, with identical results (checked by fuzz tests). Browsers compile regular expressions to machine code, so the web keeps using them.
• Flat output. Tokens are two integer lists, not a tree of objects.

🛠️ Development

This repository is a pub workspace. The Flutter version is pinned in .fvmrc (3.44.2), and the justfile runs everything through FVM:

just setup            # install the pinned SDK and fetch dependencies
just check            # format check, analyze, all tests (VM and JS)
just devices          # list available devices
just example-flutter  # run the Flutter example; asks which device to use
just example-web      # run the Flutter example in Chrome
just example-dart     # run the pure-Dart example
just bench            # benchmark

Run just with no arguments to list every command.

Note: with Xcode 27, Flutter 3.44.2 (the pinned version) fails to build for iOS, and to build macOS apps in profile or release mode, such as the frame benchmark. Its architecture check rejects Xcode 27's lipo output. Flutter 3.47 builds both. Web, Android and macOS debug builds work with the pin.

Built by Val

val_highlight is part of Val, the live visual layer for AI agents.

💬 Community

Issues and pull requests are welcome. See CONTRIBUTING.md for setup, tests and what we look for in a pull request.

📄 License

BSD 3-Clause — see LICENSE.