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The default quote delimiters can be changed with the builtin
changequote
:
This sets start as the new begin-quote delimiter and end as the new end-quote delimiter. If both arguments are missing, the default quotes (
`
and'
) are used. If start is void, then quoting is disabled. Otherwise, if end is missing or void, the default end-quote delimiter ('
) is used. The quote delimiters can be of any length.The expansion of
changequote
is void.
changequote(`[', `]') => define([foo], [Macro [foo].]) => foo =>Macro foo.
The quotation strings can safely contain eight-bit characters. If no single character is appropriate, start and end can be of any length. Other implementations cap the delimiter length to five characters, but GNU has no inherent limit.
changequote(`[[[', `]]]') => define([[[foo]]], [[[Macro [[[[[foo]]]]].]]]) => foo =>Macro [[foo]].
Calling changequote
with start as the empty string will
effectively disable the quoting mechanism, leaving no way to quote text.
However, using an empty string is not portable, as some other
implementations of m4
revert to the default quoting, while others
preserve the prior non-empty delimiter. If start is not empty,
then an empty end will use the default end-quote delimiter of
`'', as otherwise, it would be impossible to end a quoted string.
Again, this is not portable, as some other m4
implementations
reuse start as the end-quote delimiter, while others preserve the
previous non-empty value. Omitting both arguments restores the default
begin-quote and end-quote delimiters; fortunately this behavior is
portable to all implementations of m4
.
define(`foo', `Macro `FOO'.') => changequote(`', `') => foo =>Macro `FOO'. `foo' =>`Macro `FOO'.' changequote(`,) => foo =>Macro FOO.
There is no way in m4
to quote a string containing an unmatched
begin-quote, except using changequote
to change the current
quotes.
If the quotes should be changed from, say, `[' to `[[',
temporary quote characters have to be defined. To achieve this, two
calls of changequote
must be made, one for the temporary quotes
and one for the new quotes.
Macros are recognized in preference to the begin-quote string, so if a
prefix of start can be recognized as part of a potential macro
name, the quoting mechanism is effectively disabled. Unless you use
changeword
(see Changeword), this means that start
should not begin with a letter, digit, or `_' (underscore).
However, even though quoted strings are not recognized, the quote
characters can still be discerned in macro expansion and in trace
output.
define(`echo', `$@') => define(`hi', `HI') => changequote(`q', `Q') => q hi Q hi =>q HI Q HI echo(hi) =>qHIQ changequote => changequote(`-', `EOF') => - hi EOF hi => hi HI changequote => changequote(`1', `2') => hi1hi2 =>hi1hi2 hi 1hi2 =>HI hi
Quotes are recognized in preference to argument collection. In particular, if start is a single `(', then argument collection is effectively disabled. For portability with other implementations, it is a good idea to avoid `(', `,', and `)' as the first character in start.
define(`echo', `$#:$@:') => define(`hi', `HI') => changequote(`(',`)') => echo(hi) =>0::hi changequote => changequote(`((', `))') => echo(hi) =>1:HI: echo((hi)) =>0::hi changequote => changequote(`,', `)') => echo(hi,hi)bye) =>1:HIhibye:
If end is a prefix of start, the end-quote will be recognized in preference to a nested begin-quote. In particular, changing the quotes to have the same string for start and end disables nesting of quotes. When quote nesting is disabled, it is impossible to double-quote strings across macro expansions, so using the same string is not done very often.
define(`hi', `HI') => changequote(`""', `"') => ""hi"""hi" =>hihi ""hi" ""hi" =>hi hi ""hi"" "hi" =>hi" "HI" changequote => `hi`hi'hi' =>hi`hi'hi changequote(`"', `"') => "hi"hi"hi" =>hiHIhi
It is an error if the end of file occurs within a quoted string.
`hello world' =>hello world `dangling quote ^D error-->m4:stdin:2: ERROR: end of file in string