Why can't a Regex object be immutable?

[Shriramana Sharma via Digitalmars-d-learn]

Hello. With this code:

import std.stdio, std.regex;
void main()
{
    immutable numbers = regex(r"\d+");
    foreach (match; "a1b2c3d4e5".matchAll(numbers))
	writeln(match[0]);
}

compiling gives the error:

<src>(4): Error: cannot implicitly convert expression (regex("\\d+", "")) of 
type Regex!char to immutable(Regex!char)
<src>(5): Error: template std.regex.matchAll cannot deduce function from 
argument types !()(string, immutable(Regex!char)), candidates are:
/usr/include/dmd/phobos/std/regex/package.d(859):        
std.regex.matchAll(R, RegEx)(R input, RegEx re) if (isSomeString!R && 
is(RegEx == Regex!(BasicElementOf!R)))
/usr/include/dmd/phobos/std/regex/package.d(867):        
std.regex.matchAll(R, String)(R input, String re) if (isSomeString!R && 
isSomeString!String)
/usr/include/dmd/phobos/std/regex/package.d(874):        
std.regex.matchAll(R, RegEx)(R input, RegEx re) if (isSomeString!R && 
is(RegEx == StaticRegex!(BasicElementOf!R)))

If I use `auto` all is fine. Why is it impossible for a Regex object to be 
`immutable`?

-- 
Shriramana Sharma, Penguin #395953