What is and how is defined a Text Structure or Schema?

About

When you find a structure in a text, there is a language.

It can be:

a loosely language such as natural language
or a structured computer language (from markup language to procedural one)

Schema

The schema of text is expressed via one or many regular Expressions.

A regular expression is implemented as finite automata that's why when parsing any type of text (xml, json,..) with a library you are going from state to state.

Text Parser

A text parser is implemented:

via a grammar that define Regular Expressions in its lexer/parser rules
or manually where the rules are written at hand in the code for more grain control.

Document

When the string describes a computer format, the string/text is called a document