| Rfc | 1896 |
|---|
| Title | The text/enriched MIME Content-type |
|---|
| Author | P. Resnick, A. Walker |
|---|
| Date | February
1996 |
|---|
| Format: | TXT, PS, HTML |
|---|
| Obsoletes | RFC1523, RFC1563 |
|---|
| Status: | INFORMATIONAL |
|---|
|
Network Working Group P. Resnick
Request for Comments: 1896 QUALCOMM
Obsoletes: 1523, 1563 A. Walker
Category: Informational InterCon
February 1996
The text/enriched MIME Content-type
Status of this Memo
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
Abstract
MIME [RFC-1521] defines a format and general framework for the
representation of a wide variety of data types in Internet mail. This
document defines one particular type of MIME data, the text/enriched
MIME type. The text/enriched MIME type is intended to facilitate the
wider interoperation of simple enriched text across a wide variety of
hardware and software platforms. This document is only a minor
revision to the text/enriched MIME type that was first described in
[RFC-1523] and [RFC-1563], and is only intended to be used in the
short term until other MIME types for text formatting in Internet
mail are developed and deployed.
The text/enriched MIME type
In order to promote the wider interoperability of simple formatted
text, this document defines an extremely simple subtype of the MIME
content-type "text", the "text/enriched" subtype. The content-type
line for this type may have one optional parameter, the "charset"
parameter, with the same values permitted for the "text/plain" MIME
content-type.
The text/enriched subtype was designed to meet the following
criteria:
1. The syntax must be extremely simple to parse, so that even
teletype-oriented mail systems can easily strip away the
formatting information and leave only the readable text.
2. The syntax must be extensible to allow for new formatting
commands that are deemed essential for some application.
3. If the character set in use is ASCII or an 8-bit ASCII superset,
then the raw form of the data must be readable enough to be
largely unobjectionable in the event that it is displayed on the
screen of the user of a non-MIME-conformant mail reader.
4. The capabilities must be extremely limited, to ensure that it can
represent no more than is likely to be representable by the
user's primary word processor. While this limits what can be
sent, it increases the likelihood that what is sent can be
properly displayed.
There are other text formatting standards which meet some of these
criteria. In particular, HTML and SGML have come into widespread use
on the Internet. However, there are two important reasons that this
document further promotes the use of text/enriched in Internet mail
over other such standards:
1. Most MIME-aware Internet mail applications are already able to
either properly format text/enriched mail or, at the very least,
are able to strip out the formatting commands and display the
readable text. The same is not true for HTML or SGML.
2. The current RFC on HTML [RFC-1866] and Internet Drafts on SGML
have many features which are not necessary for Internet mail, and
are missing a few capabilities that text/enriched already has.
For these reasons, this document is promoting the use of
text/enriched until other Internet standards come into more
widespread use. For those who will want to use HTML, Appendix B of
this document contains a very simple C program that converts
text/enriched to HTML 2.0 described in [RFC-1866].
Syntax
The syntax of "text/enriched" is very simple. It represents text in a
single character set--US-ASCII by default, although a different
character set can be specified by the use of the "charset" parameter.
(The semantics of text/enriched in non-ASCII character sets are
discussed later in this document.) All characters represent
themselves, with the exception of the "<" character (ASCII 60), which
is used to mark the beginning of a formatting command. A literal
less-than sign ("<") can be represented by a sequence of two such
characters, "<<".
Formatting instructions consist of formatting commands surrounded by
angle brackets ("<>", ASCII 60 and 62). Each formatting command may
be no more than 60 characters in length, all in US-ASCII, restricted
to the alphanumeric and hyphen ("-") characters. Formatting commands
may be preceded by a solidus ("/", ASCII 47), making them negations,
and such negations must always exist to balance the initial opening
commands. Thus, if the formatting command "<bold>" appears at some
point, there must later be a "</bold>" to balance it. (NOTE: The 60
character limit on formatting commands does NOT include the "<", ">",
or "/" characters that might be attached to such commands.)
Formatting commands are always case-insensitive. That is, "bold" and
"BoLd" are equivalent in effect, if not in good taste.
Line break rules
Line breaks (CRLF pairs in standard network representation) are
handled specially. In particular, isolated CRLF pairs are translated
into a single SPACE character. Sequences of N consecutive CRLF pairs,
however, are translated into N-1 actual line breaks. This permits
long lines of data to be represented in a natural looking manner
despite the frequency of line-wrapping in Internet mailers. When
preparing the data for mail transport, isolated line breaks should be
inserted wherever necessary to keep each line shorter than 80
characters. When preparing such data for presentation to the user,
isolated line breaks should be replaced by a single SPACE character,
and N consecutive CRLF pairs should be presented to the user as N-1
line breaks.
Thus text/enriched data that looks like this:
This is
a single
line
This is the
next line.
This is the
next section.
should be displayed by a text/enriched interpreter as follows:
This is a single line
This is the next line.
This is the next section.
The formatting commands, not all of which will be implemented by all
implementations, are described in the following sections.
Formatting Commands
The text/enriched formatting commands all begin with <commandname>
and end with </commandname>, affecting the formatting of the text
between those two tokens. The commands are described here, grouped
according to type.
Parameter Command
Some of the formatting commands may require one or more associated
parameters. The "param" command is a special formatting command used
to include these parameters.
Param
Marks the affected text as command parameters, to be
interpreted or ignored by the text/enriched interpreter,
but not to be shown to the reader. The "param" command
always immediately follows some other formatting command,
and the parameter data indicates some additional
information about the formatting that is to be done. The
syntax of the parameter data (whatever appears between
the initial "<param>" and the terminating "</param>") is
defined for each command that uses it. However, it is
always required that the format of such data must not
contain nested "param" commands, and either must not use
the "<" character or must use it in a way that is
compatible with text/enriched parsing. That is, the end
of the parameter data should be recognizable with either
of two algorithms: simply searching for the first
occurrence of "</param>" or parsing until a balanced
"</param>" command is found. In either case, however, the
parameter data should not be shown to the human reader.
Font-Alteration Commands
The following formatting commands are intended to alter the font in
which text is displayed, but not to alter the indentation or
justification state of the text:
Bold
causes the affected text to be in a bold font. Nested
bold commands have the same effect as a single bold
command.
Italic
causes the affected text to be in an italic font. Nested
italic commands have the same effect as a single italic
command.
Underline
causes the affected text to be underlined. Nested
underline commands have the same effect as a single
underline command.
Fixed
causes the affected text to be in a fixed width font.
Nested fixed commands have the same effect as a single
fixed command.
FontFamily
causes the affected text to be displayed in a specified
typeface. The "fontfamily" command requires a parameter
that is specified by using the "param" command. The
parameter data is a case-insensitive string containing
the name of a font family. Any currently available font
family name (e.g. Times, Palatino, Courier, etc.) may be
used. This includes font families defined by commercial
type foundries such as Adobe, BitStream, or any other
such foundry. Note that implementations should only use
the general font family name, not the specific font name
(e.g. use "Times", not "TimesRoman" nor
"TimesBoldItalic"). When nested, the inner "fontfamily"
command takes precedence. Also note that the "fontfamily"
command is advisory only; it should not be expected that
other implementations will honor the typeface information
in this command since the font capabilities of systems
vary drastically.
Color
causes the affected text to be displayed in a specified
color. The "color" command requires a parameter that is
specified by using the "param" command. The parameter
data can be one of the following:
red
blue
green
yellow
cyan
magenta
black
white
or an RGB color value in the form:
####,####,####
where '#' is a hexadecimal digit '0' through '9', 'A'
through 'F', or 'a' through 'f'. The three 4-digit
hexadecimal values are the RGB values for red, green, and
blue respectively, where each component is expressed as
an unsigned value between 0 (0000) and 65535 (FFFF). The
default color for the message is unspecified, though
black is a common choice in many environments. When
nested, the inner "color" command takes precedence.
Smaller
causes the affected text to be in a smaller font. It is
recommended that the font size be changed by two points,
but other amounts may be more appropriate in some
environments. Nested smaller commands produce ever
smaller fonts, to the limits of the implementation's
capacity to reasonably display them, after which further
smaller commands have no incremental effect.
Bigger
causes the affected text to be in a bigger font. It is
recommended that the font size be changed by two points,
but other amounts may be more appropriate in some
environments. Nested bigger commands produce ever bigger
fonts, to the limits of the implementation's capacity to
reasonably display them, after which further bigger
commands have no incremental effect.
While the "bigger" and "smaller" operators are effectively inverses,
it is not recommended, for example, that "<smaller>" be used to end
the effect of "<bigger>". This is properly done with "</bigger>".
Since the capabilities of implementations will vary, it is to be
expected that some implementations will not be able to act on some of
the font-alteration commands. However, an implementation should still
display the text to the user in a reasonable fashion. In particular,
the lack of capability to display a particular font family, color, or
other text attribute does not mean that an implementation should fail
to display text.
Fill/Justification/Indentation Commands
Initially, text/enriched text is intended to be displayed fully
filled (that is, using the rules specified for replacing CRLF pairs
with spaces or removing them as appropriate) with appropriate kerning
and letter-tracking, and using the maximum available margins as suits
the capabilities of the receiving user agent software.
The following commands alter that state. Each of these commands force
a line break before and after the formatting environment if there is
not otherwise a line break. For example, if one of these commands
occurs anywhere other than the beginning of a line of text as
presented, a new line is begun.
Center
causes the affected text to be centered.
FlushLeft
causes the affected text to be left-justified with a
ragged right margin.
FlushRight
causes the affected text to be right-justified with a
ragged left margin.
FlushBoth
causes the affected text to be filled and padded so as to
create smooth left and right margins, i.e., to be fully
justified.
ParaIndent
causes the running margins of the affected text to be
moved in. The recommended indentation change is the width
of four characters, but this may differ among
implementations. The "paraindent" command requires a
parameter that is specified by using the "param" command.
The parameter data is a comma-seperated list of one or
more of the following:
Left
causes the running left margin to be moved to the
right.
Right
causes the running right margin to be moved to the
left.
In
causes the first line of the affected paragraph to
be indented in addition to the running margin. The
remaining lines remain flush to the running margin.
Out
causes all lines except for the first line of the
affected paragraph to be indented in addition to the
running margin. The first line remains flush to the
running margin.
Nofill
causes the affected text to be displayed without filling.
That is, the text is displayed without using the rules
for replacing CRLF pairs with spaces or removing
consecutive sequences of CRLF pairs. However, the current
state of the margins and justification is honored; any
indentation or justification commands are still applied
to the text within the scope of the "nofill".
The "center", "flushleft", "flushright", and "flushboth" commands are
mutually exclusive, and, when nested, the inner command takes
precedence.
The "nofill" command is mutually exclusive with the "in" and "out"
parameters of the "paraindent" command; when they occur in the same
scope, their behavior is undefined.
The parameter data for the "paraindent" command may contain multiple
occurances of the same parameter (i.e. "left", "right", "in", or
"out"). Each occurance causes the text to be further indented in the
manner indicated by that parameter. Nested "paraindent" commands
cause the affected text to be further indented according to the
parameters. Note that the "in" and "out" parameters for "paraindent"
are mutually exclusive; when they appear together or when nested
"paraindent" commands contain both of them, their behavior is
undefined.
For purposes of the "in" and "out" parameters, a paragraph is defined
as text that is delimited by line breaks after applying the rules for
replacing CRLF pairs with spaces or removing consecutive sequences of
CRLF pairs. For example, within the scope of an "out", the line
following each CRLF is made flush with the running margin, and
subsequent lines are indented. Within the scope of an "in", the first
line following each CRLF is indented, and subsequent lines remain
flush to the running margin.
Whether or not text is justified by default (that is, whether the
default environment is "flushleft", "flushright", or "flushboth") is
unspecified, and depends on the preferences of the user, the
capabilities of the local software and hardware, and the nature of
the character set in use. On systems where full justification is
considered undesirable, the "flushboth" environment may be identical
to the default environment. Note that full justification should never
be performed inside of "center", "flushleft", "flushright", or
"nofill" environments. Note also that for some non-ASCII character
sets, full justification may be fundamentally inappropriate.
Note that [RFC-1563] defined two additional indentation commands,
"Indent" and "IndentRight". These commands did not force a line
break, and therefore their behavior was unpredictable since they
depended on the margins and character sizes that a particular
implementation used. Therefore, their use is deprecated and they
should be ignored just as other unrecognized commands.
Markup Commands
Commands in this section, unlike the other text/enriched commands are
declarative markup commands. Text/enriched is not intended as a full
markup language, but instead as a simple way to represent common
formatting commands. Therefore, markup commands are purposely kept to
a minimum. It is only because each was deemed so prevalent or
necessary in an e-mail environment that these particular commands
have been included at all.
Excerpt
causes the affected text to be interpreted as a textual
excerpt from another source, probably a message being
responded to. Typically this will be displayed using
indentation and an alternate font, or by indenting lines
and preceding them with "> ", but such decisions are up
to the implementation. Note that as with the
justification commands, the excerpt command implicitly
begins and ends with a line break if one is not already
there. Nested "excerpt" commands are acceptable and
should be interpreted as meaning that the excerpted text
was excerpted from yet another source. Again, this can be
displayed using additional indentation, different colors,
etc.
Optionally, the "excerpt" command can take a parameter by
using the "param" command. The format of the data is
unspecified, but it is intended to uniquely identify the
text from which the excerpt is taken. With this
information, an implementation should be able to uniquely
identify the source of any particular excerpt, especially
if two or more excerpts in the message are from the same
source, and display it in some way that makes this
apparent to the user.
Lang
causes the affected text to be interpreted as belonging
to a particular language. This is most useful when two
different languages use the same character set, but may
require a different font or formatting depending on the
language. For instance, Chinese and Japanese share
similar character glyphs, and in some character sets like
UNICODE share common code points, but it is considered
very important that different fonts be used for the two
languages, especially if they appear together, so that
meaning is not lost. Also, language information can be
used to allow for fancier text handling, like spell
checking or hyphenation.
The "lang" command requires a parameter using the "param"
command. The parameter data can be any of the language
tags specified in [RFC-1766], "Tags for the
Identification of Languages". These tags are the two
letter language codes taken from [ISO-639] or can be
other language codes that are registered according to the
instructions in the Langauge Tags RFC. Consult that memo
for further information.
Balancing and Nesting of Formatting Commands
Pairs of formatting commands must be properly balanced and nested.
Thus, a proper way to describe text in bold italics is:
<bold><italic>the-text</italic></bold>
or, alternately,
<italic><bold>the-text</bold></italic>
but, in particular, the following is illegal text/enriched:
<bold><italic>the-text</bold></italic>
The nesting requirement for formatting commands imposes a slightly
higher burden upon the composers of text/enriched bodies, but
potentially simplifies text/enriched displayers by allowing them to
be stack-based. The main goal of text/enriched is to be simple enough
to make multifont, formatted email widely readable, so that those
with the capability of sending it will be able to do so with
confidence. Thus slightly increased complexity in the composing
software was deemed a reasonable tradeoff for simplified reading
software. Nonetheless, implementors of text/enriched readers are
encouraged to follow the general Internet guidelines of being
conservative in what you send and liberal in what you accept. Those
implementations that can do so are encouraged to deal reasonably with
improperly nested text/enriched data.
Unrecognized formatting commands
Implementations must regard any unrecognized formatting command as
"no-op" commands, that is, as commands having no effect, thus
facilitating future extensions to "text/enriched". Private extensions
may be defined using formatting commands that begin with "X-", by
analogy to Internet mail header field names.
In order to formally define extended commands, a new Internet
document should be published.
White Space in Text/enriched Data
No special behavior is required for the SPACE or TAB (HT) character.
It is recommended, however, that, at least when fixed-width fonts are
in use, the common semantics of the TAB (HT) character should be
observed, namely that it moves to the next column position that is a
multiple of 8. (In other words, if a TAB (HT) occurs in column n,
where the leftmost column is column 0, then that TAB (HT) should be
replaced by 8-(n mod 8) SPACE characters.) It should also be noted
that some mail gateways are notorious for losing (or, less commonly,
adding) white space at the end of lines, so reliance on SPACE or TAB
characters at the end of a line is not recommended.
Initial State of a text/enriched interpreter
Text/enriched is assumed to begin with filled text in a variable-
width font in a normal typeface and a size that is average for the
current display and user. The left and right margins are assumed to
be maximal, that is, at the leftmost and rightmost acceptable
positions.
Non-ASCII character sets
One of the great benefits of MIME is the ability to use different
varieties of non-ASCII text in messages. To use non-ASCII text in a
message, normally a charset parameter is specified in the Content-
type line that indicates the character set being used. For purposes
of this RFC, any legal MIME charset parameter can be used with the
text/enriched Content-type. However, there are two difficulties that
arise with regard to the text/enriched Content-type when non-ASCII
text is desired. The first problem involves difficulties that occur
when the user wishes to create text which would normally require
multiple non-ASCII character sets in the same text/enriched message.
The second problem is an ambiguity that arises because of the
text/enriched use of the "<" character in formatting commands.
Using multiple non-ASCII character sets
Normally, if a user wishes to produce text which contains characters
from entirely different character sets within the same MIME message
(for example, using Russian Cyrillic characters from ISO 8859-5 and
Hebrew characters from ISO 8859-8), a multipart message is used.
Every time a new character set is desired, a new MIME body part is
started with different character sets specified in the charset
parameter of the Content-type line. However, using multiple character
sets this way in text/enriched messages introduces problems. Since a
change in the charset parameter requires a new part, text/enriched
formatting commands used in the first part would not be able to apply
to text that occurs in subsequent parts. It is not possible for
text/enriched formatting commands to apply across MIME body part
boundaries.
[RFC-1341] attempted to get around this problem in the now obsolete
text/richtext format by introducing different character set
formatting commands like "iso-8859-5" and "us-ascii". But this, or
even a more general solution along the same lines, is still
undesirable: It is common for a MIME application to decide, for
example, what character font resources or character lookup tables it
will require based on the information provided by the charset
parameter of the Content-type line, before it even begins to
interpret or display the data in that body part. By allowing the
text/enriched interpreter to subsequently change the character set,
perhaps to one completely different from the charset specified in the
Content-type line (with potentially much different resource
requirements), too much burden would be placed on the text/enriched
interpreter itself.
Therefore, if multiple types of non-ASCII characters are desired in a
text/enriched document, one of the following two methods must be
used:
1. For cases where the different types of non-ASCII text can be
limited to their own paragraphs with distinct formatting, a
multipart message can be used with each part having a
Content-Type of text/enriched and a different charset parameter.
The one caveat to using this method is that each new part must
start in the initial state for a text/enriched document. That
means that all of the text/enriched commands in the preceding
part must be properly balanced with ending commands before the
next text/enriched part begins. Also, each text/enriched part
must begin a new paragraph.
2. If different types of non-ASCII text are to appear in the same
line or paragraph, or if text/enriched formatting (e.g. margins,
typeface, justification) is required across several different
types of non-ASCII text, a single text/enriched body part should
be used with a character set specified that contains all of the
required characters. For example, a charset parameter of
"UNICODE-1-1-UTF-7" as specified in [RFC-1642] could be used for
such purposes. Not only does UNICODE contain all of the
characters that can be represented in all of the other registered
ISO 8859 MIME character sets, but UTF-7 is fully compatible with
other aspects of the text/enriched standard, including the use of
the "<" character referred to below. Any other character sets
that are specified for use in MIME which contain different types
of non-ASCII text can also be used in these instances.
Use of the "<" character in formatting commands
If the character set specified by the charset parameter on the
Content-type line is anything other than "US-ASCII", this means that
the text being described by text/enriched formatting commands is in a
non-ASCII character set. However, the commands themselves are still
the same ASCII commands that are defined in this document. This
creates an ambiguity only with reference to the "<" character, the
octet with numeric value 60. In single byte character sets, such as
the ISO-8859 family, this is not a problem; the octet 60 can be
quoted by including it twice, just as for ASCII. The problem is more
complicated, however, in the case of multi-byte character sets, where
the octet 60 might appear at any point in the byte sequence for any
of several characters.
In practice, however, most multi-byte character sets address this
problem internally. For example, the UNICODE character sets can use
the UTF-7 encoding which preserves all of the important ASCII
characters in their single byte form. The ISO-2022 family of
character sets can use certain character sequences to switch back
into ASCII at any moment. Therefore it is specified that, before
text/enriched formatting commands, the prevailing character set
should be "switched back" into ASCII, and that only those characters
which would be interpreted as "<" in plain text should be interpreted
as token delimiters in text/enriched.
The question of what to do for hypothetical future character sets
that do not subsume ASCII is not addressed in this memo.
Minimal text/enriched conformance
A minimal text/enriched implementation is one that converts "<<" to
"<", removes everything between a <param> command and the next
balancing </param> command, removes all other formatting commands
(all text enclosed in angle brackets), and, outside of <nofill>
environments, converts any series of n CRLFs to n-1 CRLFs, and
converts any lone CRLF pairs to SPACE.
Notes for Implementors
It is recognized that implementors of future mail systems will want
rich text functionality far beyond that currently defined for
text/enriched. The intent of text/enriched is to provide a common
format for expressing that functionality in a form in which much of
it, at least, will be understood by interoperating software. Thus, in
particular, software with a richer notion of formatted text than
text/enriched can still use text/enriched as its basic
representation, but can extend it with new formatting commands and by
hiding information specific to that software system in text/enriched
<param> constructs. As such systems evolve, it is expected that the
definition of text/enriched will be further refined by future
published specifications, but text/enriched as defined here provides
a platform on which evolutionary refinements can be based.
An expected common way that sophisticated mail programs will generate
text/enriched data is as part of a multipart/alternative construct.
For example, a mail agent that can generate enriched mail in ODA
format can generate that mail in a more widely interoperable form by
generating both text/enriched and ODA versions of the same data,
e.g.:
Content-type: multipart/alternative; boundary=foo
--foo
Content-type: text/enriched
[text/enriched version of data]
--foo Content-type: application/oda
[ODA version of data]
--foo--
If such a message is read using a MIME-conformant mail reader that
understands ODA, the ODA version will be displayed; otherwise, the
text/enriched version will be shown.
In some environments, it might be impossible to combine certain
text/enriched formatting commands, whereas in others they might be
combined easily. For example, the combination of <bold> and <italic>
might produce bold italics on systems that support such fonts, but
there exist systems that can make text bold or italicized, but not
both. In such cases, the most recently issued (innermost) recognized
formatting command should be preferred.
One of the major goals in the design of text/enriched was to make it
so simple that even text-only mailers will implement enriched-to-
plain-text translators, thus increasing the likelihood that enriched
text will become "safe" to use very widely. To demonstrate this
simplicity, an extremely simple C program that converts text/enriched
input into plain text output is included in Appendix A.
Extensions to text/enriched
It is expected that various mail system authors will desire
extensions to text/enriched. The simple syntax of text/enriched, and
the specification that unrecognized formatting commands should simply
be ignored, are intended to promote such extensions.
An Example
Putting all this together, the following "text/enriched" body
fragment:
From: Nathaniel Borenstein <nsb@bellcore.com>
To: Ned Freed <ned@innosoft.com>
Content-type: text/enriched
<bold>Now</bold> is the time for <italic>all</italic>
good men
<smaller>(and <<women>)</smaller> to
<ignoreme>come</ignoreme>
to the aid of their
<color><param>red</param>beloved</color>
country.
By the way,
I think that <paraindent><param>left</param><<smaller>
</paraindent>should REALLY be called
<paraindent><param>left</param><<tinier></paraindent>
and that I am always right.
-- the end
represents the following formatted text (which will, no doubt, look
somewhat cryptic in the text-only version of this document):
Now is the time for all good men (and <women>) to come
to the aid of their
beloved country.
By the way, I think that
<smaller>
should REALLY be called
<tinier>
and that I am always right.
-- the end
where the word "beloved" would be in red on a color display.
ti 0 Security Considerations
Security issues are not discussed in this memo, as the mechanism
raises no security issues.
Authors' Addresses
For more information, the authors of this document may be contacted
via Internet mail:
Peter W. Resnick
QUALCOMM Incorporated
6455 Lusk Boulevard
San Diego, CA 92121-2779
Phone: +1 619 587 1121
Fax: +1 619 658 2230
EMail: presnick@qualcomm.com
Amanda Walker
InterCon Systems Corporation
950 Herndon Parkway
Herndon, VA 22070
Phone: +1 703 709 5500
Fax: +1 703 709 5555
EMail: amanda@intercon.com
Acknowledgements
The authors gratefully acknowledge the input of many contributors,
readers, and implementors of the specification in this document.
Particular thanks are due to Nathaniel Borenstein, the original
author of RFC 1563.
References
[RFC-1341]
Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
Extensions): Mechanisms for Specifying and Describing the Format
of Internet Message Bodies", 06/11/1992.
[RFC-1521]
Borenstein, N., and N. Freed, "MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", 09/23/1993.
[RFC-1523]
Borenstein, N., "The text/enriched MIME Content-type",
09/23/1993.
[RFC-1563]
Borenstein, N., "The text/enriched MIME Content-type",
01/10/1994.
[RFC-1642]
Goldsmith, D., Davis, M., "UTF-7 - A Mail-Safe Transformation
Format of Unicode", 07/13/1994.
[RFC-1766]
Alvestrand, H., "Tags for the Identification of Languages",
03/02/1995.
[RFC-1866]
Berners-Lee, T., and D. Connolly, D., "Hypertext Markup Language
- 2.0", 11/03/1995.
Appendix A--A Simple enriched-to-plain Translator in C
One of the major goals in the design of the text/enriched subtype of
the text Content-Type is to make formatted text so simple that even
text-only mailers will implement enriched-to-plain-text translators,
thus increasing the likelihood that multifont text will become "safe"
to use very widely. To demonstrate this simplicity, what follows is a
simple C program that converts text/enriched input into plain text
output. Note that the local newline convention (the single character
represented by "\n") is assumed by this program, but that special
CRLF handling might be necessary on some systems.
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
main() {
int c, i, paramct=0, newlinect=0, nofill=0;
char token[62], *p;
while ((c=getc(stdin)) != EOF) {
if (c == '<') {
if (newlinect == 1) putc(' ', stdout);
newlinect = 0;
c = getc(stdin);
if (c == '<') {
if (paramct <= 0) putc(c, stdout);
} else {
ungetc(c, stdin);
for (i=0, p=token;
(c=getc(stdin)) != EOF && c != '>'; i++) {
if (i < sizeof(token)-1)
*p++ = isupper(c) ? tolower(c) : c;
}
*p = '\0';
if (c == EOF) break;
if (strcmp(token, "param") == 0)
paramct++;
else if (strcmp(token, "nofill") == 0)
nofill++;
else if (strcmp(token, "/param") == 0)
paramct--;
else if (strcmp(token, "/nofill") == 0)
nofill--;
}
} else {
if (paramct > 0)
; /* ignore params */
else if (c == '\n' && nofill <= 0) {
if (++newlinect > 1) putc(c, stdout);
} else {
if (newlinect == 1) putc(' ', stdout);
newlinect = 0;
putc(c, stdout);
}
}
}
/* The following line is only needed with line-buffering */
putc('\n', stdout);
exit(0);
}
It should be noted that one can do considerably better than this in
displaying text/enriched data on a dumb terminal. In particular, one
can replace font information such as "bold" with textual emphasis
(like *this* or _T_H_I_S_). One can also properly handle the
text/enriched formatting commands regarding indentation,
justification, and others. However, the above program is all that is
necessary in order to present text/enriched on a dumb terminal
without showing the user any formatting artifacts.
Appendix B--A Simple enriched-to-HTML Translator in C
It is fully expected that other text formatting standards like HTML
and SGML will supplant text/enriched in Internet mail. It is also
likely that as this happens, recipients of text/enriched mail will
wish to view such mail with an HTML viewer. To this end, the
following is a simple example of a C program to convert text/enriched
to HTML. Since the current version of HTML at the time of this
document's publication is HTML 2.0 defined in [RFC-1866], this
program converts to that standard. There are several text/enriched
commands that have no HTML 2.0 equivalent. In those cases, this
program simply puts those commands into processing instructions; that
is, surrounded by "<?" and ">". As in Appendix A, the local newline
convention (the single character represented by "\n") is assumed by
this program, but special CRLF handling might be necessary on some
systems.
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
main() {
int c, i, paramct=0, nofill=0;
char token[62], *p;
while((c=getc(stdin)) != EOF) {
if(c == '<') {
c = getc(stdin);
if(c == '<') {
fputs("<", stdout);
} else {
ungetc(c, stdin);
for (i=0, p=token;
(c=getc(stdin)) != EOF && c != '>'; i++) {
if (i < sizeof(token)-1)
*p++ = isupper(c) ? tolower(c) : c;
}
*p = '\0';
if(c == EOF) break;
if(strcmp(token, "/param") == 0) {
paramct--;
putc('>', stdout);
} else if(paramct > 0) {
fputs("<", stdout);
fputs(token, stdout);
fputs(">", stdout);
} else {
putc('<', stdout);
if(strcmp(token, "nofill") == 0) {
nofill++;
fputs("pre", stdout);
} else if(strcmp(token, "/nofill") == 0) {
nofill--;
fputs("/pre", stdout);
} else if(strcmp(token, "bold") == 0) {
fputs("b", stdout);
} else if(strcmp(token, "/bold") == 0) {
fputs("/b", stdout);
} else if(strcmp(token, "italic") == 0) {
fputs("i", stdout);
} else if(strcmp(token, "/italic") == 0) {
fputs("/i", stdout);
} else if(strcmp(token, "fixed") == 0) {
fputs("tt", stdout);
} else if(strcmp(token, "/fixed") == 0) {
fputs("/tt", stdout);
} else if(strcmp(token, "excerpt") == 0) {
fputs("blockquote", stdout);
} else if(strcmp(token, "/excerpt") == 0) {
fputs("/blockquote", stdout);
} else {
putc('?', stdout);
fputs(token, stdout);
if(strcmp(token, "param") == 0) {
paramct++;
putc(' ', stdout);
continue;
}
}
putc('>', stdout);
}
}
} else if(c == '>') {
fputs(">", stdout);
} else if (c == '&') {
fputs("&", stdout);
} else {
if(c == '\n' && nofill <= 0 && paramct <= 0) {
while((i=getc(stdin)) == '\n') fputs("<br>", stdout);
ungetc(i, stdin);
}
putc(c, stdout);
}
}
/* The following line is only needed with line-buffering */
putc('\n', stdout);
exit(0);
}