.Sm off

.It Cm C No \&/ Ar pattern Xo

.No \&/ Ar replacement

.No \&/ Op Cm 1g

.Xc

.Sm on

The 

.Cm C

modifier is just like the

.Cm S

modifier except that the old and new strings, instead of being

simple strings, are a regular expression (see

.Xr regex 3 )

and an

.Xr ed 1 Ns \-style

replacement string.  Normally, the first occurrence of the pattern in

each word of the value is changed.  The

.Ql 1

modifier causes the substitution to apply to at most one word; the

.Ql g

modifier causes the substitution to apply to as many instances of the

search pattern as occur in the word or words it is found in.  Note that

.Ql 1

and

.Ql g

are orthogonal; the former specifies whether multiple words are

potentially affected, the latter whether multiple substitutions can

potentially occur within each affected word.