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Syntax Highlighting

JLine provides powerful syntax highlighting capabilities through its Highlighter interface. This guide explains how to implement syntax highlighting in your JLine applications.

Basic Highlighting​

The simplest way to add highlighting is to implement the Highlighter interface:

Loading snippet: BasicHighlighterExample...

In this example, all input text is highlighted in blue. The Highlighter interface has a single method, highlight, which takes the current LineReader, the input buffer, and a list of candidates to which the highlighted versions should be added.

Keyword Highlighting​

A more useful example is highlighting specific keywords:

Loading snippet: KeywordHighlighterExample...

This example uses regular expressions to identify SQL keywords and highlights them in bold blue.

Full Syntax Highlighting​

For more complex languages, you can implement a more sophisticated highlighter:

Loading snippet: SyntaxHighlighterExample...

This example demonstrates a more complete syntax highlighter for a simple programming language, with different colors for keywords, strings, numbers, and comments.

Error Highlighting​

Highlighters can also be used to indicate errors in the input:

Loading snippet: ErrorHighlightingExample...

This example checks for balanced parentheses and highlights any unmatched parenthesis with a red background.

Best Practices​

When implementing syntax highlighting in JLine, keep these best practices in mind:

  1. Performance: Highlighting is performed on every keystroke, so keep your implementation efficient.

  2. Incremental Highlighting: For large inputs, consider implementing incremental highlighting that only processes the changed portions.

  3. Error Resilience: Your highlighter should handle invalid or incomplete input gracefully.

  4. Color Choices: Choose colors that work well on both light and dark backgrounds, or adapt based on the terminal's capabilities.

  5. Accessibility: Don't rely solely on colors for conveying information.

  6. Consistency: Use consistent highlighting styles for similar elements.

  7. Terminal Capabilities: Check the terminal's capabilities before using advanced styling features.

  8. Testing: Test your highlighter with a variety of inputs to ensure it behaves as expected.

Syntax highlighting can significantly improve the usability of your command-line application by making the structure of the input more visible and helping users identify errors more quickly.