Condition
patt
/;
test is a pattern which matches only if the evaluation of test yields True.
lhs
:>
rhs
/;
test represents a rule which applies only if the evaluation of test yields True.
lhs
:=
rhs
/;
test is a definition to be used only if test yields True.
Example: The pattern x_
/;
x
>
0 represents an expression which must be positive.
All pattern variables used in test must also appear in patt.
Example: f[x_]
:=
fp[x]
/;
x
>
1 defines a function in the case when  .
See The Mathematica Book on the web:Section 2.3.5 and Section 2.5.8.
See also: If, Switch, Which, PatternTest, Element.
Further Examples
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