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”.

   • 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.