StringCases

StringCases["string",patt]

gives a list of the substrings in "string" that match the string expression patt.

StringCases["string",lhsrhs]

gives a list of the values of rhs corresponding to the substrings that match the string expression lhs.

StringCases["string",p,n]

includes only the first n substrings that match.

StringCases["string",{p₁,p₂,…}]

gives substrings that match any of the p_i.

StringCases[{s₁,s₂,…},p]

gives the list of results for each of the s_i.

StringCases[patt]

represents an operator form of StringCases that can be applied to an expression.

Details and Options

String expressions can contain any of the objects specified in the notes for StringExpression.
With the default option setting Overlaps->False, StringCases includes only substrings that do not overlap. With Overlaps->True, it includes substrings that overlap.
With Overlaps->All, multiple substrings that match the same string expression are all included. With Overlaps->True, only the first such matching substring at a given position is included.
Setting the option IgnoreCase->True makes StringCases treat lowercase and uppercase letters as equivalent.
StringCases["string",RegularExpression["regex"]] gives substrings matching the specified regular expression.
StringCases[s,lhs:>rhs] evaluates rhs only when the pattern is found.
StringCases[patt][expr] is equivalent to StringCases[expr, patt].

Background & Context

StringCases["string",{patt₁,…,patt_k}] returns a list whose elements are the substrings of "string" matching any of the patterns patt_j. The alternative form StringCases["string",patt] is equivalent to StringCases["string",{patt}], while StringCases["string",lhsrhs] gives a list of the values of rhs corresponding to the substrings of "string" matching lhs, StringCases["string",patt,n] includes only the first n substrings that match, StringCases["string",RegularExpression["regex"]] gives substrings matching the specified regular expression regex, and StringCases[patt]["string"] represents an operator form of StringCases applied to string. StringCases automatically threads over lists, with StringCases[{"string₁",…,"string_n"},…] returning the list of results for each "string_j".
The string patterns patt_j may contain any valid StringExpression objects, such as AnyOrder, FixedOrder, Condition, Whitespace, NumberString and DatePattern.
The default behavior of StringCases is equivalent to StringCases[…,IgnoreCase→False,Overlaps→False], namely to consider lowercase and uppercase letters distinct and to omit substrings that overlap. In contrast, specifying the option IgnoreCase→True makes StringCases treat lowercase and uppercase letters as equivalent. The Overlaps option has a number of possible settings, with Overlaps->All returning all multiple substrings matching the same string expression, and Overlaps->True returning only the first matching substring at a given position.
StringCases is related to a number of other symbols. StringCount and StringPosition give the number of substrings of a given string that match a particular pattern and the positions of matching substrings, respectively, and StringCount["string",patt] and StringTake["string",StringPosition["string",patt]] are equivalent to Length[StringCases["string",patt]] and StringCases["string",patt], respectively. StringContainsQ and StringFreeQ test whether a given string contains or fails to contain a substring matching a particular pattern, respectively. In particular, StringContainsQ["string",patt] returns False if and only if StringCases["string",patt] returns the empty list {} and StringFreeQ["string",patt] returns True. StringCases is the String analog of the qualitatively similar functions TextCases and Cases and is also related to TextWords, StringReplace, StringReplaceList and StringReplacePart.

Examples

open allclose all

Basic Examples (3)

Find the substrings matching a pattern:

Return only the named wild-card character in each substring:

Use the operator form of StringCases:

Scope (8)

Use string patterns:

Use a regular expression:

Use pattern matching for dates:

Mixed regular expressions and string patterns:

Rules to extract values corresponding to matching substrings:

Include only the two first strings that match:

Occurrences in either substring:

StringCases automatically threads over lists of strings:

Options (2)

IgnoreCase (1)

Find all substrings "cat", including use of uppercase letters:

Overlaps (1)

Find all runs of two or more letters starting with the letter "a":

Allow overlaps between the substrings:

Allow multiple substrings to start at the same character as well:

Applications (3)

Extract phone numbers from a text:

Find the sequence of section numbers in the US Constitution:

Find the numbers of amendments to the Constitution:

Primes whose digits are all consecutive:

Properties & Relations (2)

StringCount gives the number of matching substrings:

The length of matching substrings:

Use StringPosition to get the position of matching substrings:

Check:

Possible Issues (1)

Use :> rather than -> if the right-hand side of a rule contains string operations:

Using -> in such cases leads to immediate evaluation and possible error messages:

Neat Examples (1)

Find the sequence of words in the US Constitution:

Total number of words:

Number of distinct words:

Logarithmic frequency distribution of words:

Ten most common words:

Introduced in 2004

(5.1)

Updated in 2015

(10.1)

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