Interpolating Registers (The GNU troff Manual)

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5.8.2 Interpolating Registers

The \n escape sequence interpolates register contents.

Escape sequence: \ni

Escape sequence: \n(id

Escape sequence: \n[ident]

Interpolate register with name ident (one-character name i, two-character name id). If the register is undefined, the formatter creates it and assigns it a value of ‘0’, and interpolates that value.⁷⁴ \n is interpreted even in copy mode (see Copy Mode).

.nr a 5
.nr as \na+\na
\n(as
 ⇒ 10

.nr a1 5
.nr ab 6
.ds str b
.ds num 1
\n[a\n[num]]
 ⇒ 5
\n[a\*[str]]
 ⇒ 6