.\"	Id: mandoc_escape.3,v 1.4 2017/07/04 23:40:01 schwarze Exp 
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.\" Copyright (c) 2014 Ingo Schwarze <schwarze@openbsd.org>
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.Dd July 4, 2017
.Dt MANDOC_ESCAPE 3
.Os
.Sh NAME
.Nm mandoc_escape
.Nd parse roff escape sequences
.Sh SYNOPSIS
.In sys/types.h
.In mandoc.h
.Ft "enum mandoc_esc"
.Fo mandoc_escape
.Fa "const char **end"
.Fa "const char **start"
.Fa "int *sz"
.Fc
.Sh DESCRIPTION
This function scans a
.Xr roff 7
escape sequence.
.Pp
An escape sequence consists of
.Bl -dash -compact -width 2n
.It
an initial backslash character
.Pq Sq \e ,
.It
a single ASCII character called the escape sequence identifier,
.It
and, with only a few exceptions, an argument.
.El
.Pp
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.
.Bl -tag -width 2n
.It \&In brackets: Ic \&[ Ns Ar argument Ns Ic \&]
The argument starts after the initial
.Sq \&[ ,
ends before the final
.Sq \&] ,
and the escape sequence ends with the final
.Sq \&] .
.It Two-character argument short form: Ic \&( Ns Ar ar
This form can only be used for arguments
consisting of exactly two characters.
It has the same effect as
.Ic \&[ Ns Ar ar Ns Ic \&] .
.It One-character argument short form: Ar a
This form can only be used for arguments
consisting of exactly one character.
It has the same effect as
.Ic \&[ Ns Ar a Ns Ic \&] .
.It Delimited form: Ar C Ns Ar argument Ns Ar C
The argument starts after the initial delimiter character
.Ar C ,
ends before the next occurrence of the delimiter character
.Ar C ,
and the escape sequence ends with that second
.Ar C .
Some escape sequences allow arbitrary characters
.Ar C
as quoting characters, some restrict the range of characters
that can be used as quoting characters.
.El
.Pp
Upon function entry,
.Fa end
is expected to point to the escape sequence identifier.
The values passed in as
.Fa start
and
.Fa sz
are ignored and overwritten.
.Pp
By design, this function cannot handle those
.Xr roff 7
escape sequences that require in-place expansion, in particular
user-defined strings
.Ic \e* ,
number registers
.Ic \en ,
width measurements
.Ic \ew ,
and numerical expression control
.Ic \eB .
These are handled by
.Fn roff_res ,
a private preprocessor function called from
.Fn roff_parseln ,
see the file
.Pa roff.c .
.Pp
The function
.Fn mandoc_escape
is used
.Bl -dash -compact -width 2n
.It
recursively by itself, because some escape sequence arguments can
in turn contain other escape sequences,
.It
for error detection internally by the
.Xr roff 7
parser part of the
.Xr mandoc 3
library, see the file
.Pa roff.c ,
.It
above all externally by the
.Xr mandoc 1
formatting modules, in particular
.Fl Tascii
and
.Fl Thtml ,
for formatting purposes, see the files
.Pa term.c
and
.Pa html.c ,
.It
and rarely externally by high-level utilities using the mandoc library,
for example
.Xr makewhatis 8 ,
to purge escape sequences from text.
.El
.Sh RETURN VALUES
Upon function return, the pointer
.Fa end
is set to the character after the end of the escape sequence,
such that the calling higher-level parser can easily continue.
.Pp
For escape sequences taking an argument, the pointer
.Fa start
is set to the beginning of the argument and
.Fa sz
is set to the length of the argument.
For escape sequences not taking an argument,
.Fa start
is set to the character after the end of the sequence and
.Fa sz
is set to 0.
Both
.Fa start
and
.Fa sz
may be
.Dv NULL ;
in that case, the argument and the length are not returned.
.Pp
For sequences taking an argument, the function
.Fn mandoc_escape
returns one of the following values:
.Bl -tag -width 2n
.It Dv ESCAPE_FONT
The escape sequence
.Ic \ef
taking an argument in standard form:
.Ic \ef[ , \ef( , \ef Ns Ar a .
Two-character arguments starting with the character
.Sq C
are reduced to one-character arguments by skipping the
.Sq C .
More specific values are returned for the most commonly used arguments:
.Bl -column "argument" "ESCAPE_FONTITALIC"
.It argument Ta return value
.It Cm R No or Cm 1 Ta Dv ESCAPE_FONTROMAN
.It Cm I No or Cm 2 Ta Dv ESCAPE_FONTITALIC
.It Cm B No or Cm 3 Ta Dv ESCAPE_FONTBOLD
.It Cm P Ta Dv ESCAPE_FONTPREV
.It Cm BI Ta Dv ESCAPE_FONTBI
.El
.It Dv ESCAPE_SPECIAL
The escape sequence
.Ic \eC
taking an argument delimited with the single quote character
and, as a special exception, the escape sequences
.Em not
having an identifier, that is, those where the argument, in standard
form, directly follows the initial backslash:
.Ic \eC' , \e[ , \e( , \e Ns Ar a .
Note that the one-character argument short form can only be used for
argument characters that do not clash with escape sequence identifiers.
.Pp
If the argument matches one of the forms described below under
.Dv ESCAPE_UNICODE ,
that value is returned instead.
.Pp
The
.Dv ESCAPE_SPECIAL
special character escape sequences can be rendered using the functions
.Fn mchars_spec2cp
and
.Fn mchars_spec2str
described in the
.Xr mchars_alloc 3
manual.
.It Dv ESCAPE_UNICODE
Escape sequences of the same format as described above under
.Dv ESCAPE_SPECIAL ,
but with an argument of the forms
.Ic u Ns Ar XXXX ,
.Ic u Ns Ar YXXXX ,
or
.Ic u10 Ns Ar XXXX
where
.Ar X
and
.Ar Y
are hexadecimal digits and
.Ar Y
is not zero:
.Ic \eC'u , \e[u .
As a special exception,
.Fa start
is set to the character after the
.Ic u ,
and the
.Fa sz
return value does not include the
.Ic u
either.
.Pp
Such Unicode character escape sequences can be rendered using the function
.Fn mchars_num2uc
described in the
.Xr mchars_alloc 3
manual.
.It Dv ESCAPE_NUMBERED
The escape sequence
.Ic \eN
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
.Dv ESCAPE_IGNORE
is returned.
.Pp
Such ASCII character escape sequences can be rendered using the function
.Fn mchars_num2char
described in the
.Xr mchars_alloc 3
manual.
.It Dv ESCAPE_OVERSTRIKE
The escape sequence
.Ic \eo
followed by an argument delimited by an arbitrary character.
.It Dv ESCAPE_IGNORE
.Bl -bullet -width 2n
.It
The escape sequence
.Ic \es
followed by an argument in standard form or by an argument delimited
by the single quote character:
.Ic \es' , \es[ , \es( , \es Ns Ar a .
As a special exception, an optional
.Sq +
or
.Sq \-
character is allowed after the
.Sq s
for all forms.
.It
The escape sequences
.Ic \eF ,
.Ic \eg ,
.Ic \ek ,
.Ic \eM ,
.Ic \em ,
.Ic \en ,
.Ic \eV ,
and
.Ic \eY
followed by an argument in standard form.
.It
The escape sequences
.Ic \eA ,
.Ic \eb ,
.Ic \eD ,
.Ic \eR ,
.Ic \eX ,
and
.Ic \eZ
followed by an argument delimited by an arbitrary character.
.It
The escape sequences
.Ic \eH ,
.Ic \eh ,
.Ic \eL ,
.Ic \el ,
.Ic \eS ,
.Ic \ev ,
and
.Ic \ex
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
.Dv ESCAPE_ERROR
is returned.
.El
.It Dv 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.
.El
.Pp
For sequences that do not take an argument, the function
.Fn mandoc_escape
returns one of the following values:
.Bl -tag -width 2n
.It Dv ESCAPE_SKIPCHAR
The escape sequence
.Qq \ez .
.It Dv ESCAPE_NOSPACE
The escape sequence
.Qq \ec .
.It Dv ESCAPE_IGNORE
The escape sequences
.Qq \ed
and
.Qq \eu .
.El
.Sh FILES
This function is implemented in
.Pa mandoc.c .
.Sh SEE ALSO
.Xr mchars_alloc 3 ,
.Xr mandoc_char 7 ,
.Xr roff 7
.Sh HISTORY
This function has been available since mandoc 1.11.2.
.Sh AUTHORS
.An Kristaps Dzonsons Aq Mt kristaps@bsd.lv
.An Ingo Schwarze Aq Mt schwarze@openbsd.org
.Sh 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.
.
.ig
For these sequences, the list given below specifies a starting string
and either the length of the argument or an ending character.
The argument starts after the starting string.
In the former case, the sequence ends with the end of the argument.
In the latter case, the argument ends before the ending character,
and the sequence ends with the ending character.
..