Go: Regular expressions

Basics

The regular expression a.b matches any string that starts with an a, ends with a b, and has a single character in between (the period matches any character).

To check if there is a substring matching a.b, write: 
matched, err := regexp.MatchString(`a.b`, "aaxbb")
fmt.Println(matched) // true
fmt.Println(err)     // nil (regexp is valid)
To check if a full string matches a.b, anchor the start and the end:
  • the caret ^ matches the beginning of a text or line,
  • the dollar sign $ matches the end of a text.
matched, _ := regexp.MatchString(`^a.b$`, "aaxbb")
fmt.Println(matched) // false
Similarly, we can check if a string starts with or ends with a pattern by using only the start or end anchor.
For more complicated queries, compile the regular expression to create a Regexp object. There are two options: 
re, err := regexp.Compile(`regexp`) // error if regexp invalid
re := regexp.MustCompile(`regexp`) // panic if regexp invalid

It's convenient to use raw strings when writing regular expressions; both ordinary string literals and regular expressions use backslashes for special characters.

Cheat sheet

Choice and grouping

Regexp Meaning
xy x followed by y
x|y x or y, prefer x
xy|z same as (xy)|z
xy* same as x(y*)

Repetition

Regexp Meaning
x* zero or more x, prefer more
x*? prefer fewer
x+ one or more x, prefer more
x+? prefer fewer
x? zero or one x, prefer one
x?? prefer zero
x{n} exactly n x

Character classes

Expression Meaning
. any character
[ab] the character a or b
[^ab] any character except a or b
[a-z] any character from a to z
[a-z0-9] any character from a to z or 0 to 9
\d a digit: [0-9]
\D a non-digit: [^0-9]
\s a whitespace character: [\t\n\f\r ]
\S a non-whitespace character: [^\t\n\f\r ]
\w a word character: [0-9A-Za-z_]
\W a non-word character: [^0-9A-Za-z_]
\p{Greek} Unicode character class*
\pN one-letter name
\P{Greek} negated Unicode character class*
\PN one-letter name

Special characters

To match a special character \^$.|?*+-[]{}() literally, escape it with a backslash. For example \{ matches an opening brace symbol. Other escape sequences are:

Symbol Meaning
\t horizontal tab = \011
\n newline = \012
\f form feed = \014
\r carriage return = \015
\v vertical tab = \013
\123 octal character code (up to three digits)
\x7F hex character code (exactly two digits)

Text boundary anchors

Symbol Matches
\A at beginning of text
^ at beginning of text or line
$ at end of text
\z
\b at ASCII word boundary
\B not at ASCII word boundary

Code examples

First match

Use the FindString method to find the text of the first match. If there is no match, the return value is an empty string.

re := regexp.MustCompile(`foo.?`)
fmt.Printf("%q\n", re.FindString("seafood fool")) // "food"
fmt.Printf("%q\n", re.FindString("meat"))         // ""

Location

Use the FindStringIndex method to find loc, the location of the first match, in a string s. The match is at s[loc[0]:loc[1]]. A return value of nil indicates no match.

re := regexp.MustCompile(`ab?`)
fmt.Println(re.FindStringIndex("tablett"))    // [1 3]
fmt.Println(re.FindStringIndex("foo") == nil) // true

All matches

Use the FindAllString method to find the text of all matches. A return value of nil indicates no match.

The method takes an integer argument n; if n >= 0, the function returns at most n matches.

re := regexp.MustCompile(`a.`)
fmt.Printf("%q\n", re.FindAllString("paranormal", -1)) // ["ar" "an" "al"]
fmt.Printf("%q\n", re.FindAllString("paranormal", 2))  // ["ar" "an"]
fmt.Printf("%q\n", re.FindAllString("graal", -1))      // ["aa"]
fmt.Printf("%q\n", re.FindAllString("none", -1))       // [] (nil slice)

Replace

Use the ReplaceAllString method to replace the text of all matches. It returns a copy, replacing all matches of the regexp with a replacement string.

re := regexp.MustCompile(`ab*`)
fmt.Printf("%q\n", re.ReplaceAllString("-a-abb-", "T")) // "-T-T-"

Split

Use the Split method to slice a string into substrings separated by the regexp. It returns a slice of the substrings between those expression matches. A return value of nil indicates no match.

The method takes an integer argument n; if n >= 0, the function returns at most n matches.

a := regexp.MustCompile(`a`)
fmt.Printf("%q\n", a.Split("banana", -1)) // ["b" "n" "n" ""]
fmt.Printf("%q\n", a.Split("banana", 0))  // [] (nil slice)
fmt.Printf("%q\n", a.Split("banana", 1))  // ["banana"]
fmt.Printf("%q\n", a.Split("banana", 2))  // ["b" "nana"]

zp := regexp.MustCompile(`z+`)
fmt.Printf("%q\n", zp.Split("pizza", -1)) // ["pi" "a"]
fmt.Printf("%q\n", zp.Split("pizza", 0))  // [] (nil slice)
fmt.Printf("%q\n", zp.Split("pizza", 1))  // ["pizza"]
fmt.Printf("%q\n", zp.Split("pizza", 2))  // ["pi" "a"]

More functions: There are 16 functions following the naming pattern

Find(All)?(String)?(Submatch)?(Index)?

For example: Find, FindAllString, FindStringIndex, …

Implementation

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