XSL 2.0: Format String with _ to CamelCase - xslt

Question

I am using XSLT (2.0) as a java code generator. At the moment I have a XML which describes a database table and I want to generate the entity class for it.
The column names of the table are always lowercase with _ between the words.
Example: bat_valid_from
I want to rename it in the Java class to camelcase with first letter lowercase
Example: batValidFrom
Because I need this quiet often in my codeGen I like to have a function for it.
But I only could achieve this with two sub functions.
<xsl:function name="local:VarName">
<xsl:param name="columnName"/>
<xsl:value-of select="lower-case(substring($columnName,1,1))"/>
<xsl:value-of select="substring(local:VarName_sub($columnName),2)"/>
</xsl:function>
<xsl:function name="local:VarName_sub">
<xsl:param name="columnName"/>
<xsl:value-of select="local:VarName_sub_sub($columnName)"/>
</xsl:function>
<xsl:function name="local:VarName_sub_sub">
<xsl:param name="columnName"/>
<xsl:for-each select="tokenize($columnName, '_')">
<xsl:value-of select="upper-case(substring(.,1,1))"/>
<xsl:value-of select="substring(.,2)"/>
</xsl:for-each>
</xsl:function>
Maybe someone has an idea to simplify this?
Without the sub functions I get the following error:
A sequence of more than one item is not allowed as the first argument of fn:substring()
PS: I haven't posted the whole code to shorten the question

Answer 1

XSLT/XPath 2.0 has supports for expressions. You could do this:
string-join(
for $part in tokenize($input, '_')
return concat(
upper-case(substring($part, 1, 1)),
substring($part, 2)
)
, '')
with $input set to 'bat_valid_from', this expression would produce 'BatValidFrom'.
I'm leaving lower-casing (or not upper-casing) the initial letter as an exercise.

Answer 2

With the hint from Tomalak i was able to make all in one function.
Maybe not light weighted but works like a charm.
<xsl:function name="local:VarName">
<xsl:param name="columnName"/>
<xsl:value-of select="
concat(
lower-case(substring($columnName, 1, 1)),
substring(string-join(for $word in tokenize($columnName, '_')
return concat(
upper-case(substring($word, 1, 1)),
substring($word, 2)), '')
, 2))" />
</xsl:function>