XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (639 page)

consists of two pieces,
[a-z]
and
[0-9]
, and it matches any string that consists of a letter in the range
[a-z]
followed by a digit in the range
[0-9]
.

A
piece
is an
atom
, optionally followed by a
quantifier
. Quantifiers are described in the next section.

Unlike content models in XML Schema, there are no rules preventing ambiguities or backtracking in a regular expression. It is perfectly legal to have a regex with two branches that both match the same string, or to have two branches that start with the same characters, for example
abc|abd
.

Quantifiers

Quantifiers indicate whether, and how often, a construct may be repeated. The syntax is as follows:

Construct	Syntax
quantifier	indicator ? ?
indicator	? | * | + | ( { quantity } )
quantity	quantRange | quantMin | quantExact
quantRange	quantExact , quantExact
quantMin	quantExact ,