5.6.1 Control Characters

The mechanism of using roff’s control characters to invoke requests and call macros was introduced in Requests and Macros. Control characters are recognized only at the beginning of an input line, or at the beginning of the branch of a control structure request; see Conditionals and Loops.

A few requests cause a break implicitly; use the no-break control character to prevent the break. Break suppression is its sole behavioral distinction. Employing the no-break control character to invoke requests that don’t cause breaks is harmless but poor style. See Manipulating Filling and Adjustment.

The control ‘.’ and no-break control ‘'’ characters can each be changed to any ordinary character⁴² with the cc and c2 requests, respectively.

Request: .cc [o]

Recognize the ordinary character o as the control character. If o is absent or invalid, the default control character ‘.’ is selected. The identity of the control character is associated with the environment (see Environments).

Request: .c2 [o]

Recognize the ordinary character o as the no-break control character. If o is absent or invalid, the default no-break control character ‘'’ is selected. The identity of the no-break control character is associated with the environment (see Environments).

When writing a macro, you might wish to know which control character was used to call it.

Register: \n[.br]

This read-only register interpolates 1 if the currently executing macro was called using the normal control character and 0 otherwise. If a macro is interpolated as a string, the .br register’s value is inherited from the context of the string interpolation. See Strings.

Use this register to reliably intercept requests that imply breaks.

.als bp*orig bp
.de bp
.  ie \\n[.br] .bp*orig
.  el          'bp*orig
..

Testing the .br register outside of a macro definition makes no sense.