how to concatenate (part of) variable to String in XSLT -

i see first couple of characters of variable 'vari' concatenated string abc=' here:

href="{concat('abc=', substring-before('vari', '='))}" 

here whole snippet:

<xsl:template match="report:subelement">     <tr>         <td>             <message>               <xsl:variable name="vari"  select="."></xsl:variable>                    <xsl:copy-of select="." />             </message>         </td>         <td>            <button type="button"  onclick="window.location.href=this.getattribute('href')"  href="{concat('abc=', substring-before('vari', '='))}" >kill thread</button>         </td>     </tr> </xsl:template> 

that trivial question learning xslt.

you quite close, but: access value of variable have use dollar ($) prefix. not put variable name in apostrophe.
therefore try:

href="{concat('abc=', substring-before($vari, '='))} 

than throw error because variable declaration not in same context usage. variable declaration has in same element or in ancestor. put declaration @ top of subelement template or in <tr element.

updated working template:

<xsl:template match=""report:subelement">     <xsl:variable name="vari"  select="."></xsl:variable>     <tr>         <td>             <message>                 <xsl:copy-of select="." />             </message>         </td>         <td>             <button type="button"  onclick="window.location.href=this.getattribute('href')"                      href="{concat('abc=', substring-before($vari, '='))}" >kill thread</button>         </td>     </tr> </xsl:template>