Question

I want to replace some HTML tags that I have in a CDATA element, but I struggle with getting the syntax in XSLT right. I am getting this error message:

net.sf.saxon.trans.XPathException: Error at character 9 in regular  expression '<img(\s+(?![^<>]*alt=['\'])[^<...': expected ()) (line 51) 

I guess it does not like the <> inside the regEx. Does anyone knows how to write this in XSLT?

Here is the regEx:

<xsl:variable name='imgTagWithoutAltAttributePattern'> <xsl:text disable-output-escaping='yes'>&lt;img(\s+(?![^&lt;&gt;]*alt=['\'])[^&lt;&gt;]+)/&gt;</xsl:text></xsl:variable> 

Answer 1

I don’t think that the escaped <> brackets are the source of the problem.

Looking at the error message, the error is at char 9, where a closing parentheses ')' is expected:

<img(\s+(?![^<>]*alt=['\'])[^<... --------^ 

As you can see, the '<>' comes out just fine. I suspect that the regex engine does not understand the regex in some other way (maybe the negative look-ahead is the problem?).

I suggest to try a simpler regex at first, breaking your original one down in different tests to single out the problem:

<img\s[^>]+/>                          // test without look-ahead <img(?=\s)[^>]+/>                      // test with positive look-ahead <img(?!\S)[^>]+/>                      // test with negative look-ahead <img((?!\S))[^>]+/>                    // negative look-ahead in parentheses  <img\s(?![^>]+alt=[''])[^>]+/>         // your intention, expressed differently 

This way you could inch your way to the cause of the error.

EDIT

By the OP’s own statement, using look-ahead in the regular expression causes the error, so obviously look-ahead is not supported by this regex engine.

To match only <img> tags that don’t contain alt attributes look-around is not absolutely required. I propose a different approach:

 <img\s(a[^l]|al[^t]|alt\s*[^=]|[^a>])*>           // literal form &lt;img\s(a[^l]|al[^t]|alt\s*[^=]|[^a&gt;])*&gt;  // XML-encoded form 

Credit for this little beast goes to: J.F. Sebastian. Here is the explanation:

 <img\s          ....... start of img tag   (             ....... start of alternatives: either     a[^l]       ....... 'a', not followed by 'l'     |           ....... or     al[^t]      ....... 'al', not followed by 't'     |           ....... or     alt\s*[^=]  ....... 'alt', not followed by an equals sign     |           ....... or     [^a>]       ....... neither 'a' nor '>'   )*            ....... end of alternatives, repeat as often as possible >               ....... end of image tag 

The standard disclaimer applies: Regex is not the best tool for processing HTML. Use at your own risk.