Seven Obscure Languages in Seven Weeks

Constructing Patterns

A general SNOBOL pattern is more complex than a mere string of characters. It’s defined recursively: it can be a simple string, the joining (concatenation) of two patterns, or a choice (alternation) between them, with the alternation operation taking precedence over concatenation.

Similar to regular expressions, SNOBOL uses a vertical bar | to denote alternation between two options. A pattern like P1 | P2 is considered a match if the subject string matches P1 or P2. The following example demonstrates how to use alternation to detect either “Alice” or “Bob” within a line of text:

LOOP line = INPUT              :F(END)
     line "Alice" | "Bob"      :F(LOOP)
     OUTPUT = "Found a match!" :(LOOP)
END

Conditional Assignment ...