[xsl] Sorting chemical formulae in XSLT 2.0

Hello,

Using Saxon 9.2 and XSLT 2.0, I am currently sorting a list of chemical 
formulae which appears in the following format:

<list>
  
<item1>(C<sub>19</sub>H<sub>22</sub>N<sub>2</sub>O)<sub>2</sub>,H<sub>2</sub>SO<sub>4</sub>,7H<sub>2</sub>O</item1>
  <item1>C<sub>4</sub>H<sub>7</sub>Cl<sub>3</sub>O<sub>2</sub></item1>
  <item1>CHCl<sub>3</sub></item1>
  <item1>CNa<sub>3</sub>O<sub>5</sub>P </item1> 
</list>

The desired sort order is:

CHCl3
CNa3O5P
C4H7Cl3O2
(C19H22N2O)2,H2SO4,7H2O

So the rules are
a. ignore brackets
b. sort letters before numbers
c. sort numbers numerically

Using the following templates, I've managed to get as far as a and b, but I 
need a little help adding c to the mix:

<xsl:template match="list">
  <xsl:for-each select="item1">
    <xsl:sort select="rps:molSort(item1)" case-order="upper-first"/>
    <xsl:copy-of select="item1"/>
  </xsl:for-each>
</xsl:template>

<xsl:function name="rps:molSort" as="xs:string">
   <xsl:param name="node"/>
   <xsl:variable name="step1" select="replace(replace($node, '\(',''), 
'\)','')"/>
   <xsl:variable name="step2" select="replace(replace($step1, '\[',''), 
'\]','')"/>
   <xsl:variable name="step3" 
select="translate($step2,'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789','0123456789aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ')"/>
   <xsl:value-of select="$step3"/>
</xsl:function>
      
This produces the following output:
CHCl3
CNa3O5P
(C19H22N2O)2,H2SO4,7H2O
C4H7Cl3O2

In other words, numbers are sorted as letters rather than numbers, so the 
subscripts go "1 10 11 2 3.." instead of "1 2 3... 10 11". I need an additional 
criterion somewhere to sort the numbers correctly but I haven't found a 
solution that works yet, so a nudge in the right direction would be great.

Thank you!


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