RFC1896
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 <[email protected]> To: Ned Freed <[email protected]> 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: [email protected]
Amanda Walker InterCon Systems Corporation 950 Herndon Parkway Herndon, VA 22070
Phone: +1 703 709 5500 Fax: +1 703 709 5555 EMail: [email protected]
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("
", stdout);
ungetc(i, stdin);
}
putc(c, stdout);
}
}
/* The following line is only needed with line-buffering */
putc('\n', stdout);
exit(0);
}
