scanfile is one of a set of commands designed to assist the
user in looking through an open file for matches against a
regular expression.

scanfile actually is the 'loop' that causes the evaluations to take place.
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Hopefully someone will add in some examples here.  In particular, an example that demonstrates how to change file lines based on regular expression hits would be appreciated.
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See also [scancontext] and [scanmatch].
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[Category Command], a part of [TclX]