ExprTk is a header-only C++ library for runtime mathematical expression parsing and evaluation. It provides a complete system for defining mathematical expressions as strings, compiling them into an internal AST representation, and evaluating them with variable values. The library supports scalar, vector, and string types, arithmetic operations, trigonometric functions, logarithms, control structures (if, for, while, switch), user-defined functions, and assignment operations within expressions. It employs sophisticated parsing techniques including constant folding, strength reduction optimizations, and runtime safety checks for vector bounds and loop execution time. The architecture consists of three main components: symbol table (for variables/constants/functions), expression (holding the parsed AST), and parser (compiling expression strings).