Unique ID and multiple classes with XPath - xslt

Question

I'm using XSLT for displaying a ul menu containing li and a.
I want the following:
Find the first li a element and add the .firstitem class.
Find the last li a element and add the .lastitem class.
Find the active li a element and add the .active class.
Add an unique ID to each li a element. (I.e. URL friendly menu text as ID).
I've managed to make step 1-3 work. Except that when I try to add the classes, it actually replaces the other classes rather than adding to them.
Here's the code:
<li>
<a>
<!-- Add .firstitem class -->
<xsl:if test="position() = 1">
<xsl:attribute name="class">firstitem</xsl:attribute>
</xsl:if>
<!-- Add .lastitem class -->
<xsl:if test="postition() = count(//Page)">
<xsl:attribute name="class">lastitem</xsl:attribute>
</xsl:if>
<!-- Add .active class -->
<xsl:if test="#Active='True'">
<xsl:attribute name="class">active</xsl:attribute>
</xsl:if>
<!-- Add link URL -->
<xsl:attribute name="href"><xsl:value-of select="#FriendlyHref" disable-output-escaping="yes"/></xsl:attribute>
<!-- Add link text -->
<xsl:value-of select="#MenuText" disable-output-escaping="yes"/>
</a>
</li>
In realtity, the a element could contain all those three classes. But as is goes through the code, it replaces everything in the class attribute. How can I add the classes instead of replacing them?
And step number 4 on my list, is to get a unique ID, preferably based on #MenuText. I know there is a replace() function, but I can't get it to work and my editor says that replace() isn't a function.
The menu item text contains spaces, dashes and other symbols that are not valid for using in the id attribute. How can I replace those symbols?

Answer 1

<a>
<xsl:attribute name="class">
<!-- Add .firstitem class -->
<xsl:if test="position() = 1">
<xsl:text> firstitem</xsl:text>
</xsl:if>
<!-- Add .lastitem class -->
<xsl:if test="postition() = count(//Page)">
<xsl:text> lastitem</xsl:text>
</xsl:if>
<!-- Add .active class -->
<xsl:if test="#Active='True'">
<xsl:text> active</xsl:text>
</xsl:if>
</xsl:attribute>
<!-- Add link URL -->
<xsl:attribute name="href"><xsl:value-of select="#FriendlyHref" disable-output-escaping="yes"/></xsl:attribute>
<!-- Add link text -->
<xsl:value-of select="#MenuText" disable-output-escaping="yes"/>
</a>
replace() is an XSLT2.0 function. When using XSLT1.0 you need a custom template to do most string manipulations.

Answer 2

I'm adding this to Martijn Laarman's answer, which covers your requirements 1-3 and has my vote:
To remove everything except a certain range of characters from a string with XSLT 1.0 (your 4th requirement), do the following.
<!-- declare at top level -->
<xsl:variable
name="validRange"
select="'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
/>
<!-- later, within a template… -->
<xsl:attribute name="id">
<xsl:value-of select="
concat(
'id_',
translate(
#MenuText,
translate(#MenuText, $validRange, ''),
''
)
)
" />
</xsl:attribute>
The inner translate() removes any valid character from #MenuText, leaving only the invalid ones. These are fed to the outer translate(), which now can remove all invalid chars from the #MenuText, whatever they might be in this instance. Only the valid chars remain.
You can make a function out of it:
<xsl:template name="HtmlIdFromString">
<xsl:param name="input" select="''" />
<xsl:value-of select="
concat('id_', translate( $input, translate($input, $validRange, ''), ''))
" />
</xsl:template>
and call it like this:
<xsl:attribute name="id">
<xsl:call-template name="HtmlIdFromString">
<xsl:with-param name="input" select="#MenuText" />
</xsl:call-template>
</xsl:attribute>

Answer 3

Use
<xsl:template match="#*">
<xsl:copy>
<xsl:apply-templates select="#*"/>
</xsl:copy>
</xsl:template>
to copy all existing attributes.
The replace() function is only supported in xslt 2.0 but i found this workaround for xslt 1.0.