## Changing the lexical structure of words

The macro changeword and all associated functionnality is experimental. It is only available if the --enable-changeword option was given to configure, at GNU m4 installation time. The functionnality might change or even go away in the future. Do not rely on it. Please direct your comments about it the same way you would do for bugs.

A file being processed by m4 is split into quoted strings, words (potential macro names) and simple tokens (any other single character). Initially a word is defined by the following regular expression:

[_a-zA-Z][_a-zA-Z0-9]*


Using changeword, you can change this regular expression. Relaxing m4's lexical rules might be useful (for example) if you wanted to apply translations to a file of numbers:

changeword([_a-zA-Z0-9]+')
define(1, 0)
=>1


Tightening the lexical rules is less useful, because it will generally make some of the builtins unavailable. You could use it to prevent accidental call of builtins, for example:

define(_indir', defn(indir'))
changeword(_[_a-zA-Z0-9]*')
esyscmd(foo)
_indir(esyscmd', ls')


Because m4 constructs its words a character at a time, there is a restriction on the regular expressions that may be passed to changeword. This is that if your regular expression accepts foo', it must also accept f' and fo'.

changeword has another function. If the regular expression supplied contains any bracketed subexpressions, then text outside the first of these is discarded before symbol lookup. So:

changecom(/*', */')
changeword(#$$[_a-zA-Z0-9]*$$')
#esyscmd(ls)


m4 now requires a #' mark at the beginning of every macro invocation, so one can use m4 to preprocess shell scripts without getting shift commands swallowed, and plain text without losing various common words.

m4's macro substitution is based on text, while TeX's is based on tokens. changeword can throw this difference into relief. For example, here is the same idea represented in TeX and m4. First, the TeX version:

\def\a{\message{Hello}}
\catcode\@=0
\catcode\\=12
=>@a
=>@bye


Then, the m4 version:

define(a, errprint(Hello')')
changeword(@$$[_a-zA-Z0-9]*$$')
=>@a


In the TeX example, the first line defines a macro a to print the message Hello'. The second line defines @ to be usable instead of \ as an escape character. The third line defines \ to be a normal printing character, not an escape. The fourth line invokes the macro a. So, when TeX is run on this file, it displays the message Hello'.

When the m4 example is passed through m4, it outputs errprint(Hello)'. The reason for this is that TeX does lexical analysis of macro definition when the macro is defined. m4 just stores the text, postponing the lexical analysis until the macro is used.

You should note that using changeword will slow m4` down by a factor of about seven.