Why is my XML processing instruction target causing an 'xX[lL]' error?

The Enigma of the Invalid Processing Instruction Target

In the realm of XML and XSLT, a cryptic error message often plagues developers: "Error: The processing instruction target matching 'xX[lL]' is not allowed." This enigmatic message can leave us scratching our heads, wondering what could possibly have gone wrong.

Delving into the Depths

The root of this error lies in the XML declaration, the first line of an XML file. The XML declaration specifies the version and encoding of the document. When this declaration appears anywhere other than at the very top of the file, Xerces-based tools send out the aforementioned error message.

Resolving the Anomaly

To rectify this issue, a thorough examination of the XML file is necessary. Three potential culprits could be lurking:

1. Visible Content Preceding XML Declaration: Any blank spaces or visible characters before the XML declaration can trigger the error. Solution: Remove all such content.
2. Invisible Content (e.g., Byte Order Mark): An invisible Byte Order Mark (BOM) can also cause this issue. Solution: Use techniques such as those from the W3C's BOM in HTML page to eliminate BOM.
3. Stray XML Declaration: If an additional XML declaration exists within the XML content, likely due to improper copying or concatenation, it must be removed. Solution: Search for XML declarations and keep only the topmost one.

Conclusion

By addressing these possibilities, developers can eradicate the "The processing instruction target matching 'xX[lL]' is not allowed" error, ensuring that their XML and XSLT documents adhere to the proper syntax.

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