Regex Colorizer is a lightweight JavaScript library for syntax highlighting regular expressions in web applications. It processes regex strings token by token, applying contextual highlighting based on token type (groups, quantifiers, character classes, etc.) and active flags (like unicode). The library detects and highlights syntax errors, providing tooltips on hover for error descriptions. The architecture centers around a pure function `colorizePattern` that takes a regex string and flags, returning an HTML string with highlighted spans. This function is used by `colorizeAll` to highlight elements in the DOM. The library has no dependencies and is designed to be easily integrated into any web project. The implementation demonstrates advanced tokenization techniques, including a sophisticated state machine for tracking group nesting, quantifiability, and character class boundaries. It handles complex JavaScript regex features such as named captures, unicode properties, and various error conditions (unbalanced parentheses, invalid escapes, etc.) with precision. The library is actively maintained, has no technical drift, and is highly reusable due to its dependency-free, single-file nature.