Fix BC number regex

* the integer part may contain non-leading zeroes
* The decimal point has to be a full stop
* the fractional part requires a leading decimal point

Author: cmb69

reference/bc/book.xml

@@ -16,7 +16,7 @@
   </para>
   <para>
    Valid (aka. well-formed) BCMath numbers are strings which match the regular expression
-   <literal>/^[+-]?[0]*[1-9]*[.]?[0-9]*$/</literal>.
+   <literal>/^[+-]?[0-9]*(\.[0-9]*)?$/</literal>.
   </para>
   <caution>
    <para>