[xsl] Katakana substitution regex

HI, I have to convert some Katakana strings from "original" to "new" 
by adding ー (#x30fc;) a pronunciation character (see 
http://www.fileformat.info/info/unicode/char/30fc/index.htm).
In Japanese, there aren't any word boundaries, so essentially all of 
my search strings are substrings of the text of the current element.
When substring "a" is followed by the character ー I do not 
want to make the replacement.
example:        ブラウザ is a search string 
but it is followed by ー already -- do nothing
When substring "a" is not followed by the character ー I want 
to make the replacement to create "a" followed by ー.
example:        ブラウザ is a search string 
but it is not followed by #x30fc; already
                add to the end to make it
                ブラウザー

If I was going to just add the ー, I was able to do that with a 
regex that contained the strings that I wanted to find by using regex 
and analyze-string, where $regexSearch contains all of my search 
Katakana strings:
                <xsl:analyze-string select="." regex="({$regexSearch})">
                    <xsl:matching-substring>
                        <xsl:value-of select="regex-group(1)"/>
                        <xsl:text>&#12540;</xsl:text>
                    </xsl:matching-substring>
                    <xsl:non-matching-substring>
                        <xsl:value-of select="."/>
                    </xsl:non-matching-substring>
                </xsl:analyze-string>
However,I can't figure out how I should fit this in to an overall 
xslt, where I need to check check ahead in the element text before I 
decide to make the substitution. Currently, if there is a 
string:                &#12502;&#12521;&#12454;&#12470;&#12540;
it becomes:     &#12502;&#12521;&#12454;&#12470;&#12540;&#12540; 
(doubling the last character).
If someone has some experience with this type of search and replace 
problem, I would appreciate some guidance.
Regards, Dorothy 

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