# String Patterns

An important feature of string manipulation functions like

StringReplace is that they handle not only literal strings but also patterns for collections of strings.

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This replaces any character by

.

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You can specify patterns for strings by using

*string expressions* that contain ordinary strings mixed with

*Mathematica* symbolic pattern objects.

s_{1}~~s_{2}~~... or StringExpression[s_{1},s_{2},...] |

| a sequence of strings and pattern objects |

String expressions.

Here is a string expression that represents the string

followed by any single character.

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This makes a replacement for each occurrence of the string pattern.

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StringMatchQ["s",patt] | test whether matches patt |

StringFreeQ["s",patt] | test whether is free of substrings matching patt |

StringCases["s",patt] | give a list of the substrings of that match patt |

StringCases["s",lhs->rhs] | replace each case of lhs by rhs |

StringPosition["s",patt] | give a list of the positions of substrings that match patt |

StringCount["s",patt] | count how many substrings match patt |

StringReplace["s",lhs->rhs] | replace every substring that matches lhs |

StringReplaceList["s",lhs->rhs] | give a list of all ways of replacing lhs |

StringSplit["s",patt] | split s at every substring that matches patt |

StringSplit["s",lhs->rhs] | split at lhs, inserting rhs in its place |

Functions that support string patterns.

This gives all cases of the pattern that appear in the string.

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This gives each character that appears after an

string.

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This gives all pairs of identical characters in the string.

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You can use all the standard

*Mathematica* pattern objects in string patterns. Single blanks (

) always stand for single characters. Double blanks (

) stand for sequences of one or more characters.

Single blank (

) stands for any single character.

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Double blank (

) stands for any sequence of one or more characters.

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Triple blank (

) stands for any sequence of zero or more characters.

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"string" | a literal string of characters |

_ | any single character |

__ | any sequence of one or more characters |

___ | any sequence of zero or more characters |

x_, x__, x___ | substrings given the name x |

x:pattern | pattern given the name x |

pattern.. | pattern repeated one or more times |

pattern... | pattern repeated zero or more times |

{patt_{1},patt_{2},...} or patt_{1}|patt_{2}|... | a pattern matching at least one of the |

patt/;cond | a pattern for which cond evaluates to True |

pattern?test | a pattern for which test yields True for each character |

Whitespace | a sequence of whitespace characters |

NumberString | the characters of a number |

charobj | an object representing a character class (see below) |

RegularExpression["regexp"] | substring matching a regular expression |

Objects in string patterns.

This splits at either a colon or semicolon.

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This finds all runs containing only

or

.

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Alternatives can be given in lists in string patterns.

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You can use standard

*Mathematica* constructs such as

Characters and

CharacterRange to generate lists of alternative characters to use in string patterns.

This gives a list of characters.

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This replaces the vowel characters.

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This gives characters in the range

through

.

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In addition to allowing explicit lists of characters,

*Mathematica* provides symbolic specifications for several common classes of possible characters in string patterns.

Specifications for classes of characters.

This picks out the digit characters in a string.

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This picks out all characters except digits.

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This picks out all runs of one or more digits.

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| |

This converts the strings to numbers.

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String patterns are often used as a way to extract structure from strings of textual data. Typically this works by having different parts of a string pattern match substrings that correspond to different parts of the structure.

This picks out each

followed by a number.

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This gives the numbers alone.

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This extracts "variables" and "values" from the string.

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In many situations, textual data may contain sequences of spaces, newlines or tabs that should be considered "whitespace", and perhaps ignored. In

*Mathematica*, the symbol

Whitespace stands for any such sequence.

This removes all whitespace from the string.

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This replaces each sequence of spaces by a single comma.

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String patterns normally apply to substrings that appear at any position in a given string. Sometimes, however, it is convenient to specify that patterns can apply only to substrings at particular positions. You can do this by including symbols such as

StartOfString in your string patterns.

Constructs representing special positions in a string.

This replaces

wherever it appears in a string.

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This replaces

only when it immediately follows the start of a string.

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This replaces all occurrences of the substring

.

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This replaces only occurrences that have a word boundary on both sides.

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String patterns allow the same kind of

and other conditions as ordinary

*Mathematica* patterns.

This gives cases of unequal successive characters in the string.

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When you give an object such as

or

e.. in a string pattern,

*Mathematica* normally assumes that you want this to match the longest possible sequence of characters. Sometimes, however, you may instead want to match the shortest possible sequence of characters. You can specify this using

Shortest[p].

Longest[p] | the longest consistent match for p (default) |

Shortest[p] | the shortest consistent match for p |

Objects representing longest and shortest matches.

The string pattern by default matches the longest possible sequence of characters.

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Shortest specifies that instead the shortest possible match should be found.

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*Mathematica* by default treats characters such

and

as distinct. But by setting the option

IgnoreCase->True in string manipulation operations, you can tell

*Mathematica* to treat all such uppercase and lowercase letters as equivalent.

Specifying case-independent string operations.

This replaces all occurrences of

, independent of case.

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In some string operations, one may have to specify whether to include overlaps between substrings. By default

StringCases and

StringCount do not include overlaps, but

StringPosition does.

This picks out pairs of successive characters, by default omitting overlaps.

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This includes the overlaps.

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Options for handling overlaps in strings.

This yields only a single match.

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This yields a succession of overlapping matches.

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This includes all possible overlapping matches.

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