groff: Leading Spaces Traps
5.24.5 Leading Spaces Traps
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-- Request: .lsm macro
-- Register: \n[lsn]
-- Register: \n[lss]
Set a leading spaces trap. 'gtroff' executes MACRO when it
encounters leading spaces in an input line; the implicit line break
that normally happens in this case is suppressed. A line
consisting of spaces only, however, is treated as an empty line,
possibly subject to an empty line macro set with the 'blm' request.
Leading spaces are removed from the input line before calling the
leading spaces macro. The number of removed spaces is stored in
register 'lsn'; the horizontal space that would be emitted if there
was no leading space macro is stored in register 'lss'. Note that
'lsn' and 'lss' are available even if no leading space macro has
been set.
The first thing a leading space macro sees is a token. However,
some escapes like '\f' or '\m' are handled on the fly (see 
Gtroff Internals, for a complete list) without creating a token
at all. Consider that a line starts with two spaces followed by
'\fIfoo'. While skipping the spaces '\fI' is handled too so that
groff's current font is properly set to 'I', but the leading space
macro only sees 'foo', without the preceding '\fI'. If the macro
should see the font escape you have to 'protect' it with something
that creates a token, for example with '\&\fIfoo'.
Info Catalog
groff: Blank Line Traps
groff: Traps
groff: End-of-input Traps