Two major problems:

• You didn't encode the character using the XML's encoding before inserting it in the XML. You didn't even check if the XML's encoding could encode the character.
• You're decoding entities that were encoded because the character they represent would break the XML if present. (e.g. &).

Your solution also has some potential bugs.

• You convert what appears to be entities in CDATA sections. (Your XML generator might not produce these.)
• You don't expand í. (Your XML generator might not produce these.)
• You don't expand í. (Your XML generator might not produce these.)
• The XML is encoded, but you try to match against it as if it was text. (You're matching ASCII character and your XML generator might always use an ASCII-derived encoding).

On the stylistic side,

• \d is way too encompassing. You want [0-9]. (I'm listing this as stylistic since it won't be an issue with valid XML.)
• You have a useless modifier on your match operator.
• You have useless escapes in your pattern.

Update: Added second major problem.

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