Gnu SED inexplicably requires quoting of carat following plain left parenthesis

[Stephen Salisbury]

 To Whom It May Concern:

There seems to me to be a problem with GNU Sed 4.07 on Debian Linux (and 
reported the same with Gnu Sed 4.0x on Cygwin32) having to do with carat 
/ circumflex characters ("^") after a plain left parenthesis.

Specifically, given the one-line input file ("input"):

a(^b

the command

$ sed "s/(^/X/" < input

does not match the "(^" and emits the input text unmodified.

But if I add a backslash before the carat:

$ sed "s/(\^/X/" < input

I get the expected output:

aXb

Given the POSIX documentation, it is my impression that "^" MAY be 
treated as a meta-character when it begins an expression block but NOT 
when it is preceded by a normal (literal) left parenthesis.

Thanks,
Stephen Salisbury <address@hidden>