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#### 10.11.10 Matching Commands

The `a m` (`calc-match`) [`match`] function takes a vector of formulas and a rewrite-rule-style pattern, and produces a vector of all formulas which match the pattern. The command prompts you to enter the pattern; as for `a r`, you can enter a single pattern (i.e., a formula with meta-variables), or a vector of patterns, or a variable which contains patterns, or you can give a blank response in which case the patterns are taken from the top of the stack. The pattern set will be compiled once and saved if it is stored in a variable. If there are several patterns in the set, vector elements are kept if they match any of the patterns.

For example, ‘`match(a+b, [x, x+y, x-y, 7, x+y+z])`’ will return ‘`[x+y, x-y, x+y+z]`’.

The `import` mechanism is not available for pattern sets.

The `a m` command can also be used to extract all vector elements which satisfy any condition: The pattern ‘`x :: x>0`’ will select all the positive vector elements.

With the Inverse flag [`matchnot`], this command extracts all vector elements which do not match the given pattern.

There is also a function ‘`matches(x, p)`’ which evaluates to 1 if expression x matches pattern p, or to 0 otherwise. This is sometimes useful for including into the conditional clauses of other rewrite rules.

The function `vmatches` is just like `matches`, except that if the match succeeds it returns a vector of assignments to the meta-variables instead of the number 1. For example, ‘`vmatches(f(1,2), f(a,b))`’ returns ‘`[a := 1, b := 2]`’. If the match fails, the function returns the number 0.

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