Open the file in emacs, cursor over to the character and use C-x =.

Oh, ok, a Perl solution. Making a few assumptions about the variable storing the string, and the text in the region of the funny character ...

use utf8;
$text_before = "abc";
$text_after  = "def";
$text =~ s/($text_before).($text_after)/$1X$2/
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