The titlecaps package (http://ctan.org/pkg/titlecaps, but note that CTAN is hosed in that it doesn't show in a search, but is actually there) does this. Having written the "horrendously time-expensive" stringstrings package (as Geoffrey correctly notes), I endeavored to do better for the situation of titling. The titlecaps package is the result.
An example in the documentation shows how the fully lower-cased source:
\documentclass{article} \usepackage{titlecaps} \parskip 1em \parindent 0em \def\bs{$\backslash$} \begin{document} \Resetlcwords \Addlcwords{for a is but and with of in as the etc on to if} \titlecap{% to know that none of the words typed in this paragraph were initially upper cased might be of interest to you. it is done to demonstrate the behavioral features of this package. first, you should know the words that i have pre-designated as lower case. they are: ``for a is but and with of in as the etc on to if.'' you can define your own list. note that punctuation, like the period following the word ``if'' did not mess up the search for lower case (nor did the quotation marks just now). punctuation which is screened out of the lower-cased word search pattern include . , : ; ( ) [ ] ? ! ` ' however, I cannot screen text braces; \{for example in\} is titled, versus (for example in), since the braces are not screened out in the search for pre-designated lower-case words like for and in. However, \texttt{\bs textnc} provides a workaround: \{\textnc{for example in}\}. titlecap will consider capitalizing following a (, [, \{, or - symbol, such as (abc-def). you can use your text\textit{\relax xx} commands, like i just did here with the prior xx, but if you want the argument of that command to not be titled, you either need, in this example, to add \textit{xx} to the lowercase word list, which you can see i did not. instead, i put ``\bs relax~xx'' as the argument, so that, in essence, the \bs relax was capitalized, not the x. Or you could use \texttt{\bs textnc} . here i demonstrate that text boldface, \textbf{as in the \bs textbf command}, also works fine, as do \texttt{texttt}, \textsl{textsl}, \textsc{textsc}, \textsf{textsf}, \textit{etc}. titlecap will work on diacritical marks, such as \"apfel, \c cacao \textit{etc.}, \scriptsize fontsize \LARGE changing commands\normalsize\unskip, as well as national symbols such as \o laf, \ae gis, and \oe dipus. unfortunately, i could not get it to work on the \aa~nor the \l~symbols. the method will work with some things in math mode, capitalizing symbols if there is a leading space, $x^2$ can become $ x^2$, and it can process but it will not capitalize the greek symbols, such as $\alpha$, and will choke on most macros, if they are not direct character expansions. Additionally, \textsf{titlecaps} also works with font changing declarations, for example, \bs itshape\bs sffamily. \itshape\sffamily you can see that it works fine. likewise, any subsequent \bs textxx command will, upon completion, return the font to its prior state, such as this \textbf{textbf of some text}. you can see that i have returned to the prior font, which was italic sans-serif. now I will return to upright roman\upshape\rmfamily. a condition that will not behave well is inner braces, such as \ttfamily \bs titlecap\{blah \{inner brace material\} blah-blah\}. \rmfamily see the section on quirks and limitations for a workaround involving \texttt{\bs textnc}. titlecap will always capitalize the first word of the argument (\textbf{even if it is on the lower-case word list}), unless \texttt{\bs titlecap} is invoked with an optional argument that is anything other than a capital p. in that case, the first word will be titled \textit{unless} it is on the lowercase word list. for example, i will do a \bs titlecap[\relax s]\{\relax a~big~man\} and get ``\titlecap[s]\textnc{a big man}'' with the ``a'' not titled. i hope this package is useful to you, but as far as using \textsf{titlecaps} on such large paragraphs\ldots \textbf{do not try this at home!}} \end{document}
can become, in just a few quick seconds of execution time, this:
oforinare not supposed to be capitalized in title case. So you will need to either maintain a very nice list of exceptions, or just ignore them. If you want to implement something like this, you may want to take a look at ctan.org/tex-archive/help/Catalogue/entries/mfirstuc.html for some ideas.:)