For developing lexers using JFlex, the JFlex Support plugin can be useful. It provides syntax highlighting and other useful features for editing JFlex files. GrammarKit plugin also has builtin JFlex support.
Note that lexers, and in particular JFlex-based lexers, need to be created in such a way that they always match the entire contents of the file, without any gaps between tokens, and generate special tokens for characters which are not valid at their location. Lexers must never abort prematurely because of an invalid character.
Finally, if the custom language employs external tools for validating files in the language (for example, uses the Xerces library for XML schema validation), it can provide an implementation of the
ExternalAnnotator interface and register it in
com.intellij.externalAnnotator extension point. The ExternalAnnotator highlighting has the lowest priority and is invoked only after all other background processing has completed. It uses the same AnnotationHolder interface for converting the output of the external tool into editor highlighting.
The plugin can also provide a configuration interface to allow the user to configure the colors used for highlighting specific items. In order to do that, it should provide an implementation of
ColorSettingPage and register it in the
com.intellij.colorSettingsPage extension point.
Please note new functionality about Language Defaults and support for additional color schemes as detailed in Color Scheme Management in Intellij IDEA 12.1+
The "Export to HTML" feature of IDEA uses the same syntax highlighting mechanism as the editor, so it will work automatically for custom languages which provide a syntax highlighter.