XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (643 page)

Atoms

Atoms are the parts of a regular expression that match specified characters. The various options are defined by the syntax below.

Construct	Syntax
atom	Char | charClass | backReference | ( ( regex ) )
Char	Any XML character except . \ ? * + | ∧ $ { } ( ) [ ]
charClass	charClassEsc | charClassExpr | . | ∧ | $
charClassExpr	[ charGroup ]
backReference	\ [1-9] [0-9]*