Basic Examples

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Get the longest common prefix of strings:
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LongestCommonPrefix
[{"the cat in the hat","the cat insists"}]
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the cat in
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LongestCommonPrefix
[{"the cat in the hat","the cat insists","the catapult is inferior to the trebuchet"}]
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the cat
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Get the longest common prefix of lists:
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LongestCommonPrefix
[{{1,2,3},{1,2,x}}]
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{1,2}
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[◼]
LongestCommonPrefix
[{{a,b,c},{a,b,1,2},{a,b,"hello"}}]
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{a,b}
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[◼]
LongestCommonPrefix
[{Fibonacci[Range[3,10]],Prime[Range[10]]}]
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{2,3,5}
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Scope

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Multiple arguments will be interpreted as a list:
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LongestCommonPrefix
["the cat in the hat","the cat insists","the catastrophe"]
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the cat
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[◼]
LongestCommonPrefix
[{1,2,3},{1,2,x}]
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{1,2}
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If given an
Association
,
LongestCommonPrefix
uses the values:
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LongestCommonPrefix
[<|"a"{1,2,3},"b"{1,2,x}|>]
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{1,2}
Out[]=

Options

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IgnoreCase

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By default,
LongestCommonPrefix
is case sensitive for strings:
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LongestCommonPrefix
["Testing","TEST STRING"]
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T
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Set
IgnoreCase

True
to treat uppercase and lowercase letters as equivalent:
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LongestCommonPrefix
["Testing","TEST STRING",IgnoreCaseTrue]
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Test
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When ignoring case, the result will match the case of the first string:
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LongestCommonPrefix
["TEST STRING","Testing","tested",IgnoreCaseTrue]
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TEST
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LongestCommonPrefix
["tested","TEST STRING","Testing",IgnoreCaseTrue]
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test
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[◼]
LongestCommonPrefix
["Testing","tested","TEST STRING",IgnoreCaseTrue]
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Test
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Method

Specify that a linear search algorithm should be used:
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LongestCommonPrefix
[{"abc","abx"},Method"LinearSearch"]
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ab
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Use a binary search:
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LongestCommonPrefix
[{"abc","abx"},Method"BinarySearch"]
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ab
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Choose a search algorithm automatically:
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LongestCommonPrefix
[{"abc","abx"},MethodAutomatic]
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ab
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If prefixes are likely to be very short relative to the sequence length, a linear search can be a little faster:
Short[strings="abc"<>#&/@RandomChoice[Alphabet[],{10000,1000}]]
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{abcswzzmxuyoucnzhhhijmu…qemiurscovcjabbseusynbzf,9998,…}
Out[]//Short=
AbsoluteTiming
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LongestCommonPrefix
[strings,Method"LinearSearch"]
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{0.0055724,abc}
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AbsoluteTiming
[◼]
LongestCommonPrefix
[strings,Method"BinarySearch"]
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{0.01693,abc}
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The worse-case performance of a linear search is much worse than a binary search however:

Applications

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Find the common root directory for a list of files:
Show relative file names:
Find the common parent directory for a list of cloud objects:

Properties and Relations

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For a single string or list, the longest common prefix is simply itself:
For strings, an empty string is returned if no prefix is found:
For lists, an empty list is returned if no prefix is found:
If an empty string appears in the list of strings, the output will always be an empty string:
If an empty list appears in the list of lists, the output will always be an empty list:

Possible Issues

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Lists and strings cannot be mixed:
Since LongestCommonPrefix can return a list or string depending on the input, at least one element must be given to resolve ambiguity:

Neat Examples

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Find the lengths of longest common prefixes in random binary sequences of length 100:
Find words that share a prefix with “BirdSay”:
BirdSay the results: