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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.

Details
Details and Options Details and Options
Examples  
Basic Examples  
Scope  
Properties & Relations  
Possible Issues  
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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.

Details

  • All pattern variables used in test must also appear in patt.
  • lhs:=Module[{vars},rhs/;test] allows local variables to be shared between test and rhs. You can use the same construction with Block and With. »

Examples

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Basic Examples  (2)

Make a definition with the condition that x should be positive:

Wolfram Language code: f[x_] := ppp[x] /; x > 0
Wolfram Language code: f[5]
Wolfram Language code: f[-6]

Replace all elements which satisfy the condition of being negative:

Wolfram Language code: {6, -7, 3, 2, -1, -2} /. x_ /; x < 0 -> w

Scope  (2)

Share a variable between a condition and function body:

Wolfram Language code: f[x_] := Module[{u}, u ^ 2 /; ((u = x - 1) > 0)]
Wolfram Language code: f[0]
Wolfram Language code: f[6]

Use Condition inside a function body to control evaluation:

Wolfram Language code: f[x_] := Module[{a}, a = Prime[10 ^ x]; (FactorInteger[a + 1]) /; a < 10 ^ 6]
Wolfram Language code: f[4]
Wolfram Language code: f[5]

Properties & Relations  (2)

Condition evaluates a Boolean expression on named parts of a pattern:

Wolfram Language code: Cases[{{a, b}, {1, 2, 3}, {{d, 6}, {d, 10}}}, {x_, y_} /; !ListQ[x] && !ListQ[y]]

PatternTest applies test functions to patterns, which need not have names:

Wolfram Language code: Cases[{{a, b}, {1, 2, 3}, {{d, 6}, {d, 10}}}, {_, _} ? VectorQ]

Use Except to effectively negate Condition:

Wolfram Language code: Replace[{1, 7, "Hi", 3, z}, Except[x_ /; NumericQ[x]] :> 0, 1]

Possible Issues  (1)

Repeated[p /; test] requires that every named pattern in p have the same value throughout the sequence:

Wolfram Language code: MatchQ[{3, 3, 3}, {(x_ /; IntegerQ[x / 3])..} ]
Wolfram Language code: MatchQ[{3, 6, 9}, {(x_ /; IntegerQ[x / 3])..} ]

The same is true of RepeatedNull:

Wolfram Language code: MatchQ[{3, 6, 9}, {(x_ /; IntegerQ[x / 3])...} ]

Use PatternTest in combination with unnamed patterns to allow a sequence of nonidentical elements:

Wolfram Language code: MatchQ[{3, 6, 9}, {(_ ? (IntegerQ[# / 3]&))..} ]
Wolfram Language code: MatchQ[{3, 6, 9}, {(_ ? (IntegerQ[# / 3]&))...} ]
Wolfram Research (1988), Condition, Wolfram Language function, https://reference.wolfram.com/language/ref/Condition.html.

Text

Wolfram Research (1988), Condition, Wolfram Language function, https://reference.wolfram.com/language/ref/Condition.html.

CMS

Wolfram Language. 1988. "Condition." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/Condition.html.

APA

Wolfram Language. (1988). Condition. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/Condition.html

BibTeX

@misc{reference.wolfram_2026_condition, author="Wolfram Research", title="{Condition}", year="1988", howpublished="\url{https://reference.wolfram.com/language/ref/Condition.html}", note=[Accessed: 12-June-2026]}

BibLaTeX

@online{reference.wolfram_2026_condition, organization={Wolfram Research}, title={Condition}, year={1988}, url={https://reference.wolfram.com/language/ref/Condition.html}, note=[Accessed: 12-June-2026]}