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7.3 Changing the comment delimiters

The default comment delimiters can be changed with the builtin macro changecom:

— Builtin: changecom ([start], [end])

This sets start as the new begin-comment delimiter and end as the new end-comment delimiter. If only one argument is provided, newline becomes the new end-comment delimiter. The comment delimiters can be of any length. Omitting the first argument, or using the empty string as the first argument, disables comments.

The expansion of changecom is void. 

     define(`comment', `COMMENT')
     =>
     # A normal comment
     =># A normal comment
     changecom(`/*', `*/')
     =>
     # Not a comment anymore
     =># Not a COMMENT anymore
     But: /* this is a comment now */ while this is not a comment
     =>But: /* this is a comment now */ while this is not a COMMENT

Note how comments are copied to the output, much as if they were quoted strings. If you want the text inside a comment expanded, quote the begin-comment delimiter.

Calling changecom without any arguments, or with an empty string for the first argument, disables the commenting mechanism completely. To restore the original comment start of `#', you must explicitly ask for it. 

     define(`comment', `COMMENT')
     =>
     changecom
     =>
     # Not a comment anymore
     =># Not a COMMENT anymore
     changecom(`#')
     =>
     # comment again
     =># comment again

The comment strings can safely contain eight-bit characters.

Comments are recognized in preference to macros. However, this is not compatible with other implementations, where macros and even quoting takes precedence over comments, so it may change in a future release. For portability, this means that start should not begin with a letter or `_' (underscore), and that neither the start-quote nor the start-comment string should be a prefix of the other. 

     define(`hi', `HI')
     =>
     changecom(`q', `Q')
     =>
     q hi Q hi
     =>q hi Q HI

Comments 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')
     =>
     changecom(`(',`)')
     =>
     echo(hi)
     =>0::(hi)
     changecom
     =>
     changecom(`((', `))')
     =>
     echo(hi)
     =>1:HI:
     echo((hi))
     =>0::((hi))
     changecom(`,', `)')
     =>
     echo(hi,hi)bye)
     =>1:HI,hi)bye:

It is an error if the end of file occurs within a comment. 

     changecom(`/*', `*/')
     =>
     /*dangling comment
     ^D
     error-->m4:stdin:1: ERROR: end of file in comment