One of the unique strengths of Mathematica's core language is its powerful and succinct—yet highly readable—symbolic pattern language. Convenient both for immediate use in individual functions, and for systematic large-scale programming, Mathematica's pattern language generalizes concepts like regular expressions to describe general patterns for arbitrary symbolic structures.
x_ — any expression, to be referred to as x
Except — anything except a specified pattern
Default — predefined default arguments for a function
StringExpression — symbolic string pattern