The wiki page Regular Expressions will take you to a place where Tcl's ARE are discussed. There are other RE flavors around - Luciano ES.
Okay then - feel free to add information here on the other RE flavors available in Tcl...
LES says that there no other RE flavors available in Tcl. Tcl only uses ARE. What I meant is that regular expressions may be construed as any one (or all) of its several variations, but Regular Expressions only discusses Tcl's ARE. I said that because this wiki discusses many things under several contexts, not necessarily that of Tcl, and I thought it would be good to note that, at least in this case, it is restricted to the context of Tcl. Anyone interested in a different or more ample discussion of Regular Expressions will have to look elsewhere. E.g. on PCRE.
RS: Well, the Wiki doesn't claim to have all info - it rather asks those in the know to contribute it :) Here's from man re_syntax:
"DIFFERENT FLAVORS OF REs
Regular expressions (`REs), as defined by POSIX, come in two flavors: extended REs (EREs) and basic REs (BREs). EREs are roughly those of the traditional egrep, while BREs are roughly those of the traditional ed. This implementation adds a third flavor, advanced REs (`AREs), basically EREs with some significant extensions.
This manual page primarily describes AREs. BREs mostly exist for backward compatibility in some old programs; they will be discussed at the end. POSIX EREs are almost an exact subset of AREs. Features of AREs that are not present in EREs will be indicated."