http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/re_syntax.htm
Many threads in the comp.lang.tcl newsgroup have inspired wonderful instances of RE explication. Michael A. Cleverly, for example, wrote a delightful one [L1 ] on "Extracting numbers from text strings, removing unwanted characters".
Tcl syntax help - Arts and crafts of Tcl-Tk programming - Category Command