#syntax-highlighting #syntax-highlighter #unicode #configurable #low-level #editor #projects


A simple, low-level, syntax highlighting library with unicode support

13 releases (7 stable)

2.0.0 Feb 24, 2024
1.2.0 May 22, 2022
1.0.3 Jul 6, 2021
1.0.0 Jan 7, 2021
0.1.0 Jan 4, 2021

#44 in Text processing

Download history 91/week @ 2024-02-17 920/week @ 2024-02-24 1197/week @ 2024-03-02 1715/week @ 2024-03-09 962/week @ 2024-03-16 1301/week @ 2024-03-23 1959/week @ 2024-03-30 2038/week @ 2024-04-06 2202/week @ 2024-04-13 1896/week @ 2024-04-20 1402/week @ 2024-04-27 1437/week @ 2024-05-04 2217/week @ 2024-05-11 1951/week @ 2024-05-18

7,188 downloads per month
Used in 3 crates (2 directly)

MIT license



Syntax highlighting for Rust applications

This is a pretty lightweight (only 3 main depedencies) and simple regex-based syntax highlighter for Rust.

I originally wrote this for my text editor, Ox. It needed a fast, configurable and optimised syntax highlighter that could easily integrate with existing projects. However, you can (and are encouraged) to use it for any project you have in mind.


  • Customisable - You can highlight almost any language by adding in custom syntax highlighting rules
  • Fast - Is reasonably fast, enough so that it won't slow your projects down, even with large files and many different rules
  • Simple - You can get highlighting code pretty quickly (see example below)
  • Incremental - As this was designed for use with a text editor, it can really quickly re-highlight code upon edit commands
  • Built in language rules - Get highlighting even faster by choosing from existing syntax rules
  • File Buffering - Synoptic doesn't need the whole file to perform a correct highlighting job, thus allowing file buffering
  • Escaping - Will handle escaping if you need it ("here is a quote: \" tada!")
  • Interpolation - Will handle interpolation if you need it ("My name is {name}, nice to meet you!")


  • Not very well established - There may be inconsistencies in the included pre-built language highlighting rules
  • Lacks understanding - This will not be able to provide very detailed syntax highlighting, as no parsing is performed
  • Interpolation is limited - You can't nest interpolated tokens like "this is { "f{ "u" }n" }"

Despite its disadvantages, if you just want a simple syntax highlighter with no frills or excess baggage, synoptic might just be your crate.


Just add it to your Cargo.toml:

synoptic = "2"
  • Construct a Highlighter instance
  • Add regular expressions and keywords to the highlighter and assign each a name
  • Use the run method to generate tokens
  • Use the line method to obtain the tokens for each line

Built-in languages

You can also use some provided syntax highlighters for various popular languages using the from_extension function. There is highly likely to be inconsistencies in the existing rules, please do open an issue if you spot any.

Currently, synoptic includes

  • Various Higher Level Languages: Python, Ruby, Lua, Perl, Java, Visual Basic, Scala
  • The C Family: C, C++, C#
  • Various Lower Level Languages: Rust, Go, Assembly
  • Web Technologies: HTML, CSS, PHP, Javascript, JSON, TypeScript
  • Mathematical Languages: MATLAB, R, Haskell, Prolog
  • Moblie Development: Kotlin, Swift, Dart
  • Markup Languages: Markdown, YAML, TOML, XML, CSV
  • Other: SQL, Bash

Open an issue if there is a language not yet supported, or if you notice any issues in the built-in syntax highlighting rules.


Here's an example of a Rust syntax highlighter, using the lliw crate.

use synoptic::{Highlighter, TokOpt};
use lliw::Fg;

// Let's use some demonstration code
pub static CODE: &str = "\
Multiline comments
Work great

pub fn main() -> bool {
    // Demonstrate syntax highlighting in Rust!
    println!(\"Full Unicode Support: 你好\");
    // Interpolation
    let name = \"peter\";
    println!(\"My name is {name}, nice to meet you!\");
    // Bye!
    return true;

fn main() {
    // Setting up the highlighter
    // The `4` here just means tabs are shown as 4 spaces
    let mut h = Highlighter::new(4);
    // Bounded tokens are multiline tokens
    // Let's define multiline comments
    // In rust, these start with /* and end with */
    // Remember to escape any regex characters (like *)
    // The false here is whether or not to allow escaping
    // When true, we ignore any end markers with a backslash in front of them
    // So, if it were true: `/* this is a comment \*/ this is still a comment */ this isn't`
    h.bounded("comment", r"/\*", r"\*/", false);
    // Now let's define a string
    // In rust, format strings can be interpolated into between {}
    // We first define the name of the token, the starting and ending pattern
    // Then the starting and ending pattern of the interpolation section
    // We also want strings to be escapable e.g. "here's a quote: \" this is still a string"
    // Hence the true
    h.bounded_interp("string", "\"", "\"", "\\{", "\\}", true);
    // Now let's define some keywords
    // These are single line snippets of text
    h.keyword("keyword", r"\b(pub|fn|bool|let|return)\b");
    // Let's get numbers being highlighted
    h.keyword("digits", r"\b\d+\.(?:\.\d+)\b");
    // ... and some remaining syntax rules
    h.keyword("comment", "(//.*)$");
    h.keyword("boolean", r"\b(true|false)\b");
    h.keyword("macros", "[a-zA-Z_]+\\!");
    h.keyword("function", r"([a-z][a-zA-Z_]*)\s*\(");
    // Now let's run the highlighter on the example code
    // The run method takes a vector of strings (for each line)
    let code = CODE
        .map(|line| line.to_string())
    // Now we're ready to go
    // Let's render the output
    for (line_number, line) in code.iter().enumerate() {
        // Line returns tokens for the corresponding line
        for token in h.line(line_number, &line) {
            // Tokens can either require highlighting or not require highlighting
            match token {
                // This is some text that needs to be highlighted
                TokOpt::Some(text, kind) => print!("{}{text}{}", colour(&kind), Fg::Reset),
                // This is just normal text with no highlighting
                TokOpt::None(text) => print!("{text}"),
        // Insert a newline at the end of every line

fn colour(name: &str) -> Fg {
    // This function will take in the function name
    // And it will output the correct foreground colour
    match name {
        "comment" => Fg::LightBlack,
        "digit" => Fg::Purple,
        "string" => Fg::Green,
        "macros" => Fg::LightPurple,
        "boolean" => Fg::Blue,
        "keyword" => Fg::Yellow,
        "function" => Fg::Red,
        _ => panic!("unknown token name"),

That will render a result similar to this (depending on your terminal's colour scheme):


MIT license to ensure that you can use it in your project

you can check the LICENSE file for more info


~54K SLoC