Help: Link from if(MATCHES) to regex specification docs

Help/command/if.rst

@@ -103,7 +103,7 @@ Possible expressions are:
 
 ``if(<variable|string> MATCHES regex)``
  True if the given string or variable's value matches the given regular
- expression.
+ expression.  See :ref:`Regex Specification` for regex format.
 
 ``if(<variable|string> LESS <variable|string>)``
  True if the given string or variable's value is a valid number and less

Help/command/string.rst

@@ -77,6 +77,8 @@ The replace expression may refer to paren-delimited subexpressions of the
 match using ``\1``, ``\2``, ..., ``\9``.  Note that two backslashes (``\\1``)
 are required in CMake code to get a backslash through argument parsing.
 
+.. _`Regex Specification`:
+
 Regex Specification
 """""""""""""""""""