For declarative minilanguages, one major question is whether or not you should use XML as a base syntax and specify your grammar as an XML document type. This may well be the right thing for elaborately structured declarative minilanguages, but the same caveats we noted in Chapter5 about the design of data-file formats apply — XML might be overkill. If you don't use XML, follow the Rule of Least Surprise by supporting the Unix conventions we described for data files (simple token-oriented syntax, supporting C backslash conventions, etc.).

If you do need a custom grammar, yacc and lex (or their local equivalent in the language you're using) should probably be your best friends, unless the grammar of your language is so simple that hand-coding a recursive-descent parser is trivial. Even then, yacc may give you better error recovery, and a yacc specification will be easier to modify as the language syntax evolves. See Chapter9 for a look at the yacc- and lex-derived tools available in different implementation languages.

Even if you decide you must implement your own syntax, consider what mileage you can get from reusing existing tools. If you need a macro facility, consider whether preprocessing with m4(1) might be the right answer — but consider the cautions in the next section first.