How to parse invalid (bad / not well-formed) XML?

That "XML" is worse than invalid – it's not well-formed; see Well Formed vs Valid XML.

An informal assessment of the predictability of the transgressions does not help. That textual data is not XML. No conformant XML tools or libraries can help you process it.

Options, most desirable first:

1. Have the provider fix the problem on their end. Demand well-formed XML. (Technically the phrase well-formed XML is redundant but may be useful for emphasis.)

2. Use a tolerant markup parser to cleanup the problem ahead of parsing as XML:

   • Standalone: xmlstarlet has robust recovering and repair capabilities ^{_{credit: RomanPerekhrest}}

     xmlstarlet fo -o -R -H -D bad.xml 2>/dev/null
     

   • Standalone and C/C++: HTML Tidy works with XML too. Taggle is a port of TagSoup to C++.

   • Python: Beautiful Soup is Python-based. See notes in the Differences between parsers section. See also answers to this question for more suggestions for dealing with not-well-formed markup in Python, including especially lxml's recover=True option. See also this answer for how to use codecs.EncodedFile() to cleanup illegal characters.

   • Java: TagSoup and JSoup focus on HTML. FilterInputStream can be used for preprocessing cleanup.

   • .NET:

     • XmlReaderSettings.CheckCharacters can be disabled to get past illegal XML character problems.
     • @jdweng notes that XmlReaderSettings.ConformanceLevel can be set to ConformanceLevel.Fragment so that XmlReader can read XML Well-Formed Parsed Entities lacking a root element.
     • @jdweng also reports that XmlReader.ReadToFollowing() can sometimes be used to work-around XML syntactical issues, but note rule-breaking warning in #3 below.
     • Microsoft.Language.Xml.XMLParser is said to be “error-tolerant”.

   • Go: Set Decoder.Strict to false as shown in this example by @chuckx.

   • PHP: See DOMDocument::$recover and libxml_use_internal_errors(true). See nice example here.

   • Ruby: Nokogiri supports “Gentle Well-Formedness”.

   • R: See htmlTreeParse() for fault-tolerant markup parsing in R.

   • Perl: See XML::Liberal, a "super liberal XML parser that parses broken XML."

3. Process the data as text manually using a text editor or programmatically using character/string functions. Doing this programmatically can range from tricky to impossible as what appears to be predictable often is not -- rule breaking is rarely bound by rules.

   • For invalid character errors, use regex to remove/replace invalid characters:

     • PHP: preg_replace('/[^\x{0009}\x{000a}\x{000d}\x{0020}-\x{D7FF}\x{E000}-\x{FFFD}]+/u', ' ', $s);
     • Ruby: string.tr("^\u{0009}\u{000a}\u{000d}\u{0020}-\u{D7FF}\u{E000‌​}-\u{FFFD}", ' ')
     • JavaScript: inputStr.replace(/[^\x09\x0A\x0D\x20-\xFF\x85\xA0-\uD7FF\uE000-\uFDCF\uFDE0-\uFFFD]/gm, '')

   • For ampersands, use regex to replace matches with &: ^{_{credit: blhsin, demo}}

     &(?!(?:#\d+|#x[0-9a-f]+|\w+);)
     

Note that the above regular expressions won't take comments or CDATA sections into account.

IMO these cases should be solved by using JSoup.

Below is a not-really answer for this specific case, but found this on the web (thanks to inuyasha82 on Coderwall). This code bit did inspire me for another similar problem while dealing with malformed XMLs, so I share it here.

Please do not edit what is below, as it is as it on the original website.

The XML format, requires to be valid a unique root element declared in the document. So for example a valid xml is:

<root>
     <element>...</element>
     <element>...</element>
</root>

But if you have a document like:

<element>...</element>
<element>...</element>
<element>...</element>
<element>...</element>

This will be considered a malformed XML, so many xml parsers just throw an Exception complaining about no root element. Etc.

In this example there is a solution on how to solve that problem and succesfully parse the malformed xml above.

Basically what we will do is to add programmatically a root element.

So first of all you have to open the resource that contains your "malformed" xml (i. e. a file):

File file = new File(pathtofile);

Then open a FileInputStream:

FileInputStream fis = new FileInputStream(file);

If we try to parse this stream with any XML library at that point we will raise the malformed document Exception.

Now we create a list of InputStream objects with three lements:

A ByteIputStream element that contains the string: "" Our FileInputStream A ByteInputStream with the string: "" So the code is:

List<InputStream> streams = 
    Arrays.asList(
        new ByteArrayInputStream("<root>".getBytes()),
    fis,
    new ByteArrayInputStream("</root>".getBytes()));

Now using a SequenceInputStream, we create a container for the List created above:

InputStream cntr = 
new SequenceInputStream(Collections.enumeration(str));

Now we can use any XML Parser library, on the cntr, and it will be parsed without any problem. (Checked with Stax library);

A standard XML parser will NEVER accept invalid XML, by design.

Your only option is to pre-process the input to remove the "predictably invalid" content, or wrap it in CDATA, prior to parsing it.