void bt_purify_string (char * string, btshort options); void bt_change_case (char transform, char * string, btshort options);
void bt_purify_string (char * string, btshort options);
``Purifies'' a "string" in the BibTeX way (usually used for generating sort keys). "string" is modified in-place. "options" is currently unused; just set it to zero for future compatibility. Purification consists of copying alphanumeric characters, converting hyphens and ties to space, copying spaces, and skipping (almost) everything else.
``Almost'' because ``special characters'' (used for accented and non-English letters) are handled specially. Recall that a BibTeX special character is any brace-group that starts at brace-depth zero whose first character is a backslash. For instance, the string
{\foo bar}Herr M\"uller went from {P{\r r}erov} to {\AA}rhus
contains two special characters: "{\foo bar}" and "\AA". Neither the "\"u" nor the "\r r" are special characters, because they are not at the right brace depth.
Special characters are handled as follows: if the control sequence (the TeX command that follows the backslash) is recognized as one of LaTeX's ``foreign letters'' ("\oe", "\ae", "\o", "\l", "\ae", "\ss", plus uppercase versions), then it is converted to a reasonable English approximation by stripping the backslash and converting the second character (if any) to lowercase; thus, "{\AA}" in the above example would become simply "Aa". All other control sequences in a special character are stripped, as are all non-alphabetic characters.
For example the above string, after ``purification,'' becomes
barHerr Muller went from Pr rerov to Aarhus
Obviously, something has gone wrong with the word "P{\r r}erov" (a town in the Czech Republic). The accented `r' should be a special character, starting at brace-depth zero. If the original string were instead
{\foo bar}Herr M\"uller went from P{\r r}erov to {\AA}rhus
then the purified result would be more sensible:
barHerr Muller went from Prerov to Aarhus
Note the use of a ``nonsense'' special character "{\foo bar}": this trick is often used to put certain text in a string solely for generating sort keys; the text is then ignored when the document is processed by TeX (as long as "\foo" is defined as a no-op TeX macro). This assumes, of course, that the output is eventually processed by TeX; if not, then this trick will backfire on you.
Also, "bt_purify_string()" is adequate for generating sort keys when you want to sort according to English-language conventions. To follow the conventions of other languages, though, a more sophisticated approach will be needed; hopefully, future versions of btparse will address this deficiency.
void bt_change_case (char transform, char * string, btshort options);
Converts a string to lowercase, uppercase, or ``non-book title capitalization'', with special attention paid to BibTeX special characters and other brace-groups. The form of conversion is selected by the single character "transform": 'u' to convert to uppercase, 'l' for lowercase, and 't' for ``title capitalization''. "string" is modified in-place, and "options" is currently unused; set it to zero for future compatibility.
Lowercase and uppercase conversion are obvious, with the proviso that text in braces is treated differently (explained below). Title capitalization simply means that everything is converted to lowercase, except the first letter of the first word, and words immediately following a colon or sentence-ending punctuation. For instance,
Flying Squirrels: Their Peculiar Habits. Part One
would be converted to
Flying squirrels: Their peculiar habits. Part one
Text within braces is handled as follows. First, in a ``special character'' (see above for definition), control sequences that constitute one of LaTeX's non-English letters are converted appropriately---e.g., when converting to lowercase, "\AE" becomes "\ae"). Any other control sequence in a special character (including accents) is preserved, and all text in a special character, regardless of depth and punctuation, is converted to lowercase or uppercase. (For ``title capitalization,'' all text in a special character is converted to lowercase.)
Brace groups that are not special characters are left completely untouched: neither text nor control sequences within non-special character braces are touched.
For example, the string
A Guide to \LaTeXe: Document Preparation ...
would, when "transform" is 't' (title capitalization), be converted to
A guide to \latexe: Document preparation ...
which is probably not the desired result. A better attempt is
A Guide to {\LaTeXe}: Document Preparation ...
which becomes
A guide to {\LaTeXe}: Document preparation ...
However, if you go back and re-read the description of "bt_purify_string()", you'll discover that "{\LaTeXe}" here is a special character, but not a non-English letter: thus, the control sequence is stripped. Thus, a sort key generated from this title would be
A Guide to Document Preparation
...oops! The right solution (and this applies to any title with a TeX command that becomes actual text) is to bury the control sequence at brace-depth two:
A Guide to {{\LaTeXe}}: Document Preparation ...