CotEditor is a Cocoa-based plain-text editor specifically for Mac OS X. The app supports syntax highlighting and it has powerful find-and-replace functionality with OniGmo.

Key Features include:

• Syntax Highlighting: Colorize more than 40 pre-installed major languages like HTML, PHP, Python, Ruby or Markdown.
• Find & Replace: Powerful find and replace using OniGmo regular expression engine.
• Setting via Click:
You can access all your settings including syntax definitions and themes from a normal preferences window.
• Auto Backup: CotEditor backups your documents automatically while editing.
• Outline Menu: Extract specified lines with the predefined syntax and you can jump to the corresponding line.
• Split Editor: Split a window into multiple panes to view different area simultaneously.
• Character Inspector:Inspect unicode character data of each selected character in your document and display them in a popover.
• Scriptable: Make your own macro in your favourite language. CotEditor supports Python, Ruby, Perl, PHP, UNIX shell or AppleScript(and JavaScript on Yosemite).
• Incompatible Characters: Check and list-up the characters in your document that cannot be converted into the desired encoding.
• CJK Language Friendly: Estimate various file encodings accurately, toggle to vertical text mode and keep its line height correctly.

Overall, CotEditor is a powerful tool that allows developers to code apps for a varied selection of platforms as it comes with support for multiple programming languages.