7.6.6 Declaring the keywords and the operators

Basically, keywords and operators are lists of rules. The syntax is:

keywords are
  rules
end keywords

or

keywords in face-keyword are
  rules
end keywords

in which case the default face is set to face-keyword.

As an example:

keywords in Keyword_strong are
  /foo*/,
  "bar" "BAR" Keyword,
  -> \rightarrow
end keywords

is valid.

The syntax for the operators is the same, and both constructs can be qualified with an optional flag, in which case they are taken into account in the heavy highlighting mode (see Pretty Printing Options).

This is an extract of the C style sheet:

optional operators are
   -> \rightarrow,
   && \wedge,
   || \vee,
   != \neq,
   == \equiv,
   # We need to protect these, so that <= is not replaced in <<=
   <<=,
   >>=,
   <= \leq,
   >= \geq,
   ! \not
end operators

Note how ‘<<=’ and ‘>>=’ are protected (there are defined to be written as is when met in the source). This is to prevent the two last characters of ‘<<=’ from being converted into a ‘less or equal’ sign.

The order in which you define the elements of a category (but the sequences) does not matter. But since a2ps sorts them at run time, it may save time if the alphabetical C-order is more or less followed.

You should be aware that when declaring a keyword with a regular expression as lhs, then a2ps automatically makes this expression matching only if there are no character of the first alphabet both just before, and just after the string.

In term of implementation, it means that

keywords are
  /foo|bar/
end keywords

is exactly the same as

operators are
  /\\b(foo|bar)\\b/
end operators

This can cause problems if you use anchors (e.g. $, or ^) in keywords: the matcher will be broken. In this particular case, define your keywords as operators, taking care of the ‘\\b’ by yourself.

See Match-word-boundary Operator in Regex manual, for details on ‘\b’.