This man page describes the [regular expression] syntax supported within Tcl.

http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/re_syntax.htm 

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If someone were so inclined, it would be useful to add, to this page,
some examples of how to use some of the more complex regular expression
syntax functions.

For instance:

   atom:   a grouping of 0 or more characters which are to be treated as one entity by subsequent metacharacters, etc.  If more than one character is to be included in an atom, then some sort of grouping syntax is needed.

   branch:   {ABC|DEF} indicates that either the ''atom'' ABC or the atom DEF should match.

   quantifier:   A metacharacter appended to a regular expression atom to indicate the number of times that atom may exist.


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Many threads in [the comp.lang.tcl newsgroup] have inspired
wonderful instances of RE explication.  [Michael A. Cleverly],
for example, wrote a delightful one
[http://groups.google.com/groups?q=re_syntax+group%3acomp.lang.tcl*&hl=en&lr=&ie=UTF-8&scoring=d&selm=Pine.LNX.4.33.0206232031300.3419-100000%40gibraltar.cleverly.com&rnum=2]
on "Extracting numbers from text strings, removing unwanted characters".
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