• → PatternTest

Condition (/;)

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Condition[pattern, expr]

pattern /; expr

places an additional constraint on pattern that only allows it to match if expr evaluates to True.

The controlling expression of a Condition can use variables from the pattern:

• • f[3] /. f[x_] /; x>0 -> t

• • f[-3] /. f[x_] /; x>0 -> t

Condition can be used in an assignment:

• • f[x_] := p[x] /; x>0

• • f[3]

• • f[-3]