MANDOC_ESCAPE(3)

NAME
mandoc_escape - parse roff escape sequences

SYNOPSIS
#include <sys/types.h>
#include <mandoc.h>
enum mandoc_esc
mandoc_escape(const char **end, const char **start, int *sz);

DESCRIPTION
This function scans a roff(7) escape sequence.

An escape sequence consists of
-   an initial backslash character (‘\’),
-   a single ASCII character called the escape sequence identifier,
-   and, with only a few exceptions, an argument.

Arguments can be given in the following forms; some escape sequence identifiers only accept some of these forms as specified below. The first three forms are called the standard forms.

<brackets:> [argument]
    The argument starts after the initial ‘[’, ends before the final ‘’], and the escape sequence ends with the final ‘’].

Two-character argument short form: (ar
    This form can only be used for arguments consisting of exactly two characters. It has the same effect as [ar].

One-character argument short form: a
    This form can only be used for arguments consisting of exactly one character. It has the same effect as [a].

Delimited form: CargumentC
    The argument starts after the initial delimiter character C, ends before the next occurrence of the delimiter character C, and the escape sequence ends with that second C. Some escape sequences allow arbitrary characters C as quoting characters, some restrict the range of characters that can be used as quoting characters.

Upon function entry, end is expected to point to the escape sequence identifier. The values passed in as start and sz are ignored and overwritten.

By design, this function cannot handle those roff(7) escape sequences that require in-place expansion, in particular user-defined strings \*, number registers \n, width measurements \w, and numerical expression control \B. These are handled by roff_res(), a private preprocessor function called from roff_parseln(), see the file roff.c.

The function mandoc_escape() is used
-   recursively by itself, because some escape sequence arguments can in turn contain other escape sequences,
-   for error detection internally by the roff(7) parser part of the mandoc(3) library, see the file roff.c,
-   above all externally by the mandoc(1) formatting modules, in particular -Tascii and -Thtml, for formatting purposes, see the files term.c and html.c,
-   and rarely externally by high-level utilities using the mandoc library, for example makewhatis(8), to purge escape sequences from text.

RETURN VALUES
Upon function return, the pointer end is set to the character after the end of the escape sequence, such that the calling higher-level parser can easily continue.

For escape sequences taking an argument, the pointer start is set to the beginning of the argument and sz is set to the length of the argument. For escape sequences not taking an argument, start is set to the character after the end of the sequence and sz is set to 0. Both start and sz may be NULL; in that case, the argument and the length are not returned.

For sequences taking an argument, the function mandoc_escape() returns one of the following values:

ESCAPE_FONT
    The escape sequence \f taking an argument in standard form: \f[, \f(, \fa. Two-character arguments starting with the character ‘C’ are reduced to one-character arguments by skipping the ‘C’. More specific values are returned for the most commonly used arguments:
    argument | return value
    R or 1 | ESCAPE_FONTROMAN
    I or 2 | ESCAPE_FONTITALIC
    B or 3 | ESCAPE_FONTBOLD
    P | ESCAPE_FONTPREV
    BI | ESCAPE_FONTBI

ESCAPE_SPECIAL
    The escape sequence \C taking an argument delimited with the single quote character and, as a special exception, the escape sequences not having an identifier, that is, those where the argument, in standard form, directly follows the initial backslash: \C', \[, \(, \a. Note that the one-character argument short form can only be used for argument characters that do not clash with escape sequence identifiers.

    If the argument matches one of the forms described below under ESCAPE_UNICODE, that value is returned instead.

    The ESCAPE_SPECIAL special character escape sequences can be rendered using the functions mchars_spec2cp() and mchars_spec2str() described in the mchars_alloc(3) manual.

ESCAPE_UNICODE
    Escape sequences of the same format as described above under ESCAPE_SPECIAL, but with an argument of the forms uXXXX, uYXXXX, or u10XXXX where X and Y are hexadecimal digits and Y is not zero: \C'u, \[u. As a special exception, start is set to the character after the u, and the sz return value does not include the u either.

    Such Unicode character escape sequences can be rendered using the function mchars_num2uc() described in the mchars_alloc(3) manual.

ESCAPE_NUMBERED
    The escape sequence \N followed by a delimited argument. The delimiter character is arbitrary except that digits cannot be used. If a digit is encountered instead of the opening delimiter, that digit is considered to be the argument and the end of the sequence, and ESCAPE_IGNORE is returned.

    Such ASCII character escape sequences can be rendered using the function mchars_num2char() described in the mchars_alloc(3) manual.

ESCAPE_OVERSTRIKE
    The escape sequence \o followed by an argument delimited by an arbitrary character.

ESCAPE_IGNORE

    •   The escape sequence \s followed by an argument in standard form or by an argument delimited by the single quote character: \s', \s[, \s(, \sa. As a special exception, an optional ‘+’ or ‘-’ character is allowed after the ‘s’ for all forms.

    •   The escape sequences \F, \g, \k, \M, \m, \n, \V, and \Y followed by an argument in standard form.

    •   The escape sequences \A, \b, \D, \R, \X, and \Z followed by an argument delimited by an arbitrary character.

    •   The escape sequences \H, \h, \L, \l, \S, \v, and \x followed by an argument delimited by a character that cannot occur in numerical expressions. However, if any character that can occur in numerical expressions is found instead of a delimiter, the sequence is considered to end with that character, and ESCAPE_ERROR is returned.

ESCAPE_ERROR
    Escape sequences taking an argument but not matching any of the above patterns. In particular, that happens if the end of the logical input line is reached before the end of the argument.

For sequences that do not take an argument, the function mandoc_escape() returns one of the following values:

ESCAPE_SKIPCHAR
    The escape sequence "\z".

ESCAPE_NOSPACE
    The escape sequence "\c".

ESCAPE_IGNORE
    The escape sequences "\d" and "\u".

FILES
This function is implemented in mandoc.c.

SEE ALSO
mchars_alloc(3), mandoc_char(7), roff(7)

HISTORY
This function has been available since mandoc 1.11.2.

AUTHORS
Kristaps Dzonsons <kristaps@bsd.lv> Ingo Schwarze <schwarze@openbsd.org>

BUGS
The function doesn't cleanly distinguish between sequences that are valid and supported, valid and ignored, valid and unsupported, syntactically invalid, or undefined. For sequences that are ignored or unsupported, it doesn't tell whether that deficiency is likely to cause major formatting problems and/or loss of document content. The function is already rather complicated and still parses some sequences incorrectly.