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It's properties files allows fine tweaks of its behavior, at a global or per language or per project level. These textual settings might be confusing for those used to preference dialogs, but proves to be powerful, flexible and fine grained.
With less than 2 MB of binary on Windows, it starts instantly.
Plus, if you don't need all the config, syntax files, blah, there's a 678k standalone .exe version. Nothing is going to beat that for lightweight and start-up times. Stick it in a folder that is already on your PATH.
Works on Windows and Linux, has a commercial port on MacOS.
Pro Built-in shell
The console window can show the result of ran commands (like build current file, reporting warnings and errors), but also accept interactive shell commands.
Pro Powerful syntax highlighting for numerous languages
Lexers providing folding and syntax highlighting are based on code, not on regular expressions.
They support context, nesting, special rules, etc.
Pro Free (except on Mac) and open source
Written in C++, with lot of contributors, both to the core and to the numerous lexers.
Based on the Scintilla source code editor, it has some advanced features like rectangular editing, simple regular expression search and replace, code folding, etc. It allows to launch a compiler or interpreter and can interpret the errors messages, jumping at the location they point to.
Lua scripting is key to SciTE's power and flexibility. The Lua scripting language can be used to perform complex text transformations. It's relatively simple syntax and large user base makes it a great choice for a scripting feature.
Con Hard to config
The configuration is mainly a file-based config, which can be unintuitive and difficult to use for new users.
Con Missing file browser
SciTE's greatest weakness is perhaps the file browser. It does not really have one, just a poor substitute which works a little bit like a terminal window with
dir commands to show the files in a directory.