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8.2 Changing the quote characters

The default quote delimiters can be changed with the builtin changequote:

Builtin: changequote ([start = ‘`] @dvar{end, '})

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 section Changing the lexical structure of words), 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:

However, if you are not worried about portability, using ‘(’ and ‘)’ as quoting characters has an interesting property—you can use it to compute a quoted string containing the expansion of any quoted text, as long as the expansion results in both balanced quotes and balanced parentheses. The trick is realizing expand uses ‘$1’ unquoted, to trigger its expansion using the normal quoting characters, but uses extra parentheses to group unquoted commas that occur in the expansion without consuming whitespace following those commas. Then _expand uses changequote to convert the extra parentheses back into quoting characters. Note that it takes two more changequote invocations to restore the original quotes. Contrast the behavior on whitespace when using ‘$*’, via quote, to attempt the same task.

changequote(`[', `]')dnl
define([a], [1, (b)])dnl
define([b], [2])dnl
define([quote], [[$*]])dnl
define([expand], [_$0(($1))])dnl
define([_expand],
  [changequote([(], [)])$1changequote`'changequote(`[', `]')])dnl
expand([a, a, [a, a], [[a, a]]])
⇒1, (2), 1, (2), a, a, [a, a]
quote(a, a, [a, a], [[a, a]])
⇒1,(2),1,(2),a, a,[a, a]

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
ifelse(`dangling quote
^D
error-->m4:stdin:1: ERROR: end of file in string

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