Parsing simple input

It is a common task to parse a small text. Such situations are always a dilemma: shall we use some third-party professional and good tools for parsing such as Bison or ANTLR, or shall we try to write it by hand using only C++ and standard library? The third-party tools are good for handling the parsing of complex texts, and it is easy to write parsers using them, but they require additional tools for creating C++ or C code from their grammar, and add more dependencies to your project.

Handwritten parsers are usually hard to maintain, but they require nothing except a C++ compiler.

Let's start from a very simple task to parse a date in the ISO format as follows:

YYYY-MM-DD 

The following are the examples of possible input:

2013-03-01 
2012-12-31  // (woo-hoo, it almost a new year!) 

We will need parser's grammar from the following link http://www...