"Regular Expression Objects" scanner method

[ast]

Hello

I am trying to understand a program which use
a method named scanner on a re compiled object.

This program is named "Simple compiler" here:
https://www.pythonsheets.com/notes/python-generator.html

But unfortunately it is not documented

nor here:
https://docs.python.org/3.7/library/re.html#regular-expression-objects

nor with help

import re
lex = re.compile("foo")
help(lex.scanner)

Help on built-in function scanner:

scanner(string, pos=0, endpos=2147483647) method of _sre.SRE_Pattern 
instance

Does anybody know where to find a doc ?