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In article <1989Aug30.133802.15579@cs.dal.ca> armstrng@cs.dal.ca
(Stan Armstrong) writes:
>I need to do some simple string manipulation in TeX.

The usual trick is to define delimited macros, e.g.,

	\def\@get#1.#2\@@{\def\@got{#1}}
	\def\namepart#1{\@get#1.\@@\@got}

then `\namepart{a.b}' expands to \@get a.b.\@@, which makes \@get's
#1 be `a' and #2 be `b.'.  If you give \namepart no `.', the second `.'
supplies the one \@get needs, so that \namepart{a} expands to \@get a.\@@,
making #1 be `a' and #2 be empty (an implicit space, I think; see
_The_TeXbook_ to be sure).

A similar (but different) trick can be used to extract the `ext' part
`b'.  It is a bit more complicated.

However:

>I would like to redefine the \input command so that an alternate file
>extension is used when the file is not found.

You cannot achieve this.  The only way to test for a file's existence
is to open it; and open always succeeds.  If the file cannot be opened,
TeX interacts with its operator to get a different name---and it will
not take `no name' for an answer!  (I consider this a bug.)

>... I need to recognize the first period which is
>not between square brackets as the beginning of the file extension since
>VMS filenames are of the form:
>
>nodename::devicename:[directory.subdirectory]filename.extension;version

Not that it will do any good (see `However' above), but \catcode tricks
can help here.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris