Aside from being useful, this code showcases several poorly-documented features of GTK 3 that may be of interest to some. Namely:

1. the API to implement custom drawing (including geometry management and invoking libcairo)
2. how to set attributes on widgets that are ignored by the CSS styler (letter-spacing, for example)
3. which properties are allowed on what widget and how to write an XML UI definition file
4. examples of queueing draw operations on a widget manually to keep all widgets up-to-date

The most interesting take-away is probably this: the GtkStyleProvider in the form of a CSS styler works very poorly. For serious applications, using libpango is a must. Also, for anyone who wishes to implement anything beyond the most basic user interfaces, custom drawing (and associated libcairo) is another must. Therefore, the GTK 3 documentation is as good as useless beyond being a reference for function declarations, signal declarations, and property lists.

Type a regular expression into the top text widget. The status box will turn green for valid regular expressions (as defined by POSIX) and red for invalid regular expressions.

Type a target string into the bottom box. All matches, as computed by regexec, will be highlighted in a green box with a blue border. The empty string is never matched.

• Add a list view that allows saving regular expressions of interest. Allow output to a text file.
• Add a Java mode which runs a Java program in daemon mode that tests regular expressions as defined in java.util.regex.
• Write a custom regcomp--one that does not allow nonsense regular expressions like "ab+*".