When using patterns to match sections of input data, you must first capture the data in pattern variables for later use. Pattern variables are assigned using the
=> symbol, and referenced later. For example, in the first
find rule in the following program the matched input data is assigned to the "found-text" pattern variable.
process submit "Mary had a little [white] lamb" find ("[" letter+ "]") => found-text output found-text find any
This program outputs "[white]".
What if you want to output only the word in the square brackets, but not the brackets themselves? Try this:
process submit "Mary had a little [white] lamb" find "[" letter+ => found-text "]" output found-text find any
This program outputs "white". Here, the pattern variable is attached only to the part of the pattern immediately preceding the pattern variable assignment. In fact, this is the default behavior of pattern variables. That's why, to make the previous example work correctly, we had to surround the three elements of the pattern with parentheses to ensure that the text matched by the whole pattern was captured.
You can have more than one pattern variable in a pattern. You can even nest them. For example:
process submit "Mary had a little [white] lamb" find ("[" => first-bracket letter+ => found-word "]" => second-bracket) => found-text output first-bracket output found-word output second-bracket output found-text find any
The output of this program would be "[white][white]". The first "[white]" is the result of the first three output actions, and the second the result of the fourth output action.
Pattern variables are "declared" in a manner slightly different from other names: they are declared in a pattern assignment embedded in a pattern (using
=>), instead of in a variable declaration.
Because pattern variables are declared and can be referenced before the local scope begins, they are treated differently than the variables declared within the local scope. In effect, they are treated as if two local scopes are defined, one within the other. The outer one defines the pattern variables, and the inner can contain local declarations to declare other kinds of shelves.
matchpattern can also be referenced in the condition which follows it. (Versions of OmniMark prior to V3 did not permit this.)
matchpattern. The declared local variable hides the pattern variable.
The other context in which patterns, and therefore pattern assignments, can occur, is the second argument of the
matches operator. This argument defines an entire local scope itself. This means that: