[pcre-dev] Error in command-line regex at offset 4: invalid …

I built pcre-8.33 with MinGW on Windows 7 using
configure --enable-pcre16 --enable-pcre32 --enable-unicode-properties

Whenever I input:
grep -ui español *.txt
I get:
pcregrep: Error in command-line regex at offset 4: invalid UTF-8 string

If I input:
grep -ui espa\xf1ol *.txt
I get results

All the files in question are in utf-8 encoding.
How can I get results without using hex. numbers?

Many thanks in advance,

--
Cesar