std.regex.replaceFirst uses
https://github.com/D-Programming-Language/phobos/blob/master/std/regex/package.d#L734
which returns the input, if no match is found. This is "safe",
because the worst thing that can happen is that you get the input
back unchanged.
std.regex.replaceFirstInto, on the other hand, uses
https://github.com/D-Programming-Language/phobos/blob/master/std/regex/package.d#L700
which may throw an exception like this:
phobos/std/regex/package.d(993): invalid submatch number 1*
Shouldn't this be reflected in the documentation? At the moment,
the documentation of replaceFirstInto says:
"A variation on replaceFirst that instead of allocating a new
string on each call outputs the result piece-wise to the sink."
But it doesn't mention the difference in behavior (i.e. return
input vs. exception). In other words, with replaceFirstInto, you
have to make sure that you can indeed replace.
The same probably goes for replaceAll / replaceAllInto.
*Exception stems from
https://github.com/D-Programming-Language/phobos/blob/master/std/regex/package.d#L962
