how to check if each char in a given string is a lette

Hello, everyone,
I have a string  like this:
C12H10N2

Using xlt and fo, I need to display all digit characters smaller and
lower than letters.
To check if a  character in the string is a digit, I have the code like
this:
<xsl:template name="formatFormula">
  <xsl:param name="formula"/>

  <xsl:if test="string-length($formula)>0">
  <xsl:variable name="firstChar">
    <xsl:value-of select="substring($formula,1,1)"/>
  </xsl:variable>

  <xsl:choose>

    <!-- ???? Not work as I want ??? -->
    <xsl:when test="translate($firstChar,'0123456789','9')='9'">

      <fo:inline font-size="8" color="red"
vertical-align="sub"><xsl:value-of select="$firstChar"/></fo:inline>
    </xsl:when>
    <xsl:otherwise>
 <fo:inline font-size="10"><xsl:value-of
select="$firstChar"/></fo:inline>
    </xsl:otherwise>
  </xsl:choose>
  </xsl:if>

  <!-- calling itself recursively -->
  <xsl:if test="string-length($formula)>1">
    <xsl:call-template name="formatFormula">
      <xsl:with-param name="formula">
        <xsl:value-of select="substring($formula, 2)"/>
      </xsl:with-param>
    </xsl:call-template>
  </xsl:if>
</xsl:template>


But the result is that only the char '0' get translate and thus get
detected as a digit. All other digits are not detected. Any better
solutions?


Thanks in advance for your help.

Biying


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