Question

Invalid characters in XML

Asked by: DASAIS01UK

Hi all,

I am developing an application using XML to talk with legacy backend (mainframe), some old mainframe application may use some invalid characters (e.g. low value x'00') such that when these characters are exist in the tag value, the xml parser will throw exception.

My question is, how many characters in XML are regarded as "invalid"? I've found some information in W3C (http://www.w3.org/TR/REC-xml/#NT-Char). It said the following:

"
Character Range
[2]    Char    ::=    #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF] /* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */
"

But I'm not quite understand for the above description.  What is the corresponding Hex value ?  Can anyone help to give me a brief explanation?


Thanks a lot!


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Asked On
2005-11-05 at 03:21:40ID21620392
Tags

xml

,

invalid

,

characters

Topic

Extensible Markup Language (XML)

Participating Experts
2
Points
250
Comments
5

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    Answers

     

    by: GertonePosted on 2005-11-05 at 07:05:11ID: 15230977

    Your question addresses multiple topics.
    I ll try to give a brief explanation of these topics

    in an XML stream are the characters allowed that are mentioned in the above definition
    #x9 is the ninth character as defined in Unicode
    The codechars you can find here
    http://www.unicode.org/Public/4.1.0/charts/CodeCharts.pdf (30 MegaByte)

    Not allowed in an XML stream are the first 32 characters in this set
    (including NUL(0), excluding "tab"(9), "linefeed" (10) and "carriage return"(13))
    This means 29 characters are not allowed
    Then there are a number of characters not allowed in the higher blocks starting at character 55296
    You can count them your self from the above definition of Char.

    The codecharts are just a mapping between a character and a numerical representation.
    How they are stored on the computer depends on the encoding.
    At this point, number of bits/bytes and byte order come into play

    When you ask "what is the corresponding Hex value"? I assume you want the encoding question answered. Well this depends on the encoding.

    At the start of your XML document you can have this string
    <?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?>
    The ISO-8859-1 encoding is the ISO standardised Latin one (Ascii). That characterset encodes 1byte, so 255 characters and these 255 characters byte numbers match the first part of the unicode standard, so that is easy

    This means that with the iso latin encoding you cannot express a "h with a ^" (character 293 or x125) except with a character entity, like this "&#x125;" or "&#293;" (using 6/7 bytes)

    If this xml declaration is not present, UTF-8 is assumed as encoding.
    UTF8 uses one byte for the first 127 characters (exact match to ascii as well) but uses two bytes for the next part (one special code byte plus another for counting:
    in iso latin 1 "é" would be one byte, 233
    in UTF-8, "é" would be two bytes

    If you are pulling information streams from a mainframe. You have to know the meaning of each byte value on the mainframe, map it to the encoding you pick for the XML
    and remove or escape the characters that are not allowed

    On a higher level an XML stream consists of markup and character data.
    Since markup uses some special characters "<", ">" and "&". These need to be escaped.
    A "<" or a "&" in a character data part makes the XML unvalid as well...

    I hope this is a start.
    Happy to provide you with more info if required

    Geert
    here you need to escape "<" by "&lt;" and "&" by "&amp;"

     

    by: pescateraPosted on 2005-11-07 at 06:24:42ID: 15238897

    Hi! take a look at

    http://www.w3schools.com/xml/xml_cdata.asp

    as you will see, portions of xml inside a cdata section will be ignored by the parser. Maybe this can helpful for you

     

    by: GertonePosted on 2005-11-07 at 06:59:07ID: 15239194

    Dasaiso1uk,

    I just sam my last line should be moved 3 lines up, editor mistake

    Pescatera,

    CDATA sections will not help for for characters not allowed in the XML datastream. Not allowed means not allowed
    CDATA sections only help for the higher level escape, as I talked about in my last paragraph

    cheers

     

    by: pescateraPosted on 2005-11-07 at 07:27:31ID: 15239442

    Hi, dasa not only you've been nasty, but a little stupid too. In no place of your answer you said anything about cdata sections, and i only thougth it could help, sorry that my comment wasn't as brilliant as yours.

     

    by: DASAIS01UKPosted on 2005-11-07 at 16:48:06ID: 15244294

    Thanks for your clear explanation!

    20120131-EE-VQP-002

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