I used to think that regexp(5) on HPUX was the worst prose ever. This pair of paragraphs is representative of it:
«The concatenation of patterns matching a single character is a valid pattern that matches the concatenation of the single characters or collating elements matched by each of the concatenated patterns. For example, the pattern a[bc] matches the string ab and ac.
The concatenation of one or more patterns matching a single character with one or more asterisks is a valid pattern. In such patterns, each asterisk matches a string of zero or more characters, up to the first character that matches the character following the asterisk in the pattern.»
But Specification of Abstract Syntax Notation One (ASN.1), ITU-T Recommendation X.208 is absolutely breathtaking. Here is but one paragraph, from page 48:
The resulting type and value of an instance of use of the new value notation is determined by the value (and the type of the value) finally assigned to the distinguished local value reference identified by the keyword VALUE, according to the processing of the macrodefinition for the new type notation followed by that for the new value notation.
-- [as a PDF]
The SGML Handbook could ALMOST stand a chance at besting that, except that it's not really even in a human language -- it's just lots of weird alphanumeric cartouches, with some commentary in English. It ends up looking like a PostScript error. Here is one of the more coherent pages, which I scanned in so I could show it to the incredulous. Look -- IF YOU DARE!