Regular expression to match “>”, “<”, “&” chars that appear inside XML nodes

Question

I\'m trying to write a regular expression using the PCRE library in PHP.

I need a regex to match only &, > and < cha

Answer 1

What you have there is not, of course, XML. In XML, the characters '<' and '&' may not occur (unescaped) inside text: only inside a comment, CDATA section, or processing instruction. Actually, '>' can occur in text, except as part of the string ']]>'. In well-formed XML, literal '<' and '&' characters signal the start of markup: '<' signals the start of a start tag, end tag, or empty element tag, and '&' signals the start of an entity reference. In both these cases, the next character may NOT be whitespace. So using an RE like Robusto's suggestion would find all such occurrences. You might also need to catch corner cases like '<<', '<\', or '&<'. In this case you don't need to try to parse your input, an RE will work fine.

If the source contains strings like ' Name ::= NameStartChar (NameChar)*

Then you have more of a problem. You are going to have to (try to) parse your input as if it were real XML, and detect the error cases of malformed Names, non-matching start & end tags, malformed attributes, and undefined entity references (to name a few). Unfortunately the error condition isn't guaranteed to happen at the location of the error.

Your best bet may be to use an RE to catch 90% of the error and fix the rest manually. You need to look for a '<' or '&' followed by anything other than a NameStartChar