A set of tools for building parsers using the Go programming language.
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A set of parsing tools for Go inspired by Sprache.


Parsers first need to read data to see if the data matches a pattern. If the data doesn't match, then the parser will need to move back to the end position of the last successful parse to try a different pattern.

Just storing everything in RAM works if your file sizes are small, but your process uses a lot of RAM.

Writing to a file to do this would mean seeking on disk, potentially making the performance suffer.

Instead, the Stream type provides a way of reading runes (characters) from an input bufio.Reader into a cache in RAM. Once a token has been consumed by the parser, the consumed bytes are discarded. The amount of RAM consumed will depend on the parser that uses it.

The Stream type implements the parse.Input interface:

// Input represents the input to a parser.
type Input interface {
	// Collect collects all of the string data parsed so far and returns it, then starts a new collection
	// from the current position in the input.
	Collect() string
	// Advance advances the input by a single rune and consumes it.
	Advance() (rune, error)
	// Retreat retreats the input position by a single rune and unconsumes it.
	Retreat() (rune, error)
	// Peek returns the next rune from the input without consuming it.
	Peek() (rune, error)
	// Position returns the line and column number of the current position within the stream.
	Position() (line, column int)
	// Index returns the current index of the parser input.
	Index() int64

Parser Functions

Parser functions provide a way of matching patterns in a given input. They are designed to be able to be composed together to make more complex operations.

The examples directory contains several examples of composing the primitive functions.


  • Any
    • Parse any of the provided parse functions, or roll back.
  • AnyRune
    • Parse any rune.
  • AtLeast
    • Parse the provided function at least the number of times specified, or roll back.
  • AtMost
    • Parse the provided function at least once, and at most the number of times specified, or roll back.
  • Letter
    • Parse any letter in the Unicode Letter range or roll back.
  • Many
    • Parse the provided parse function a number of times or roll back.
  • Optional
    • Attempt to parse, but don't roll back if a match isn't found.
  • Or
    • Return the first successful result of the provided parse functions, or roll back.
  • Rune
    • Parse the specified rune (character) or fallback.
  • RuneIn
    • Parse a rune from the input stream if it's in the specified string, or roll back.
  • RuneInRanges
    • Parse a rune from the input stream if it's in the specified Unicode ranges, or roll back.
  • RuneNotIn
    • Parse a rune from the input stream if it's not in the specified string, or roll back.
  • RuneWhere
    • Parse a rune from the input stream if the predicate function passed in succeeds, or roll back.
  • String
    • Parse a string from the input stream if it exactly matches the provided string, or roll back.
  • StringUntil
    • Parse a string from the input stream until the end of the file or the specified until parser is matched.
  • Then
    • Return the results of the first and second parser passed through the combiner function which converts the two results into a single output (a map / reduce operation), or roll back if either doesn't match.
  • Times
    • Parse using the specified function a set number of times or roll back.
  • ZeroToNine
    • Parse a rune from the input stream if it's within the set of 1234567890.


Using the Or function to parse either 'A' or 'B':

parser := parse.Or(parse.Rune('A'), parse.Rune('B'))

matchesA := parser(input.NewFromString("A")).Success // true
matchesB := parser(input.NewFromString("B")).Success // true
matchesC := parser(input.NewFromString("C")).Success // false

fmt.Println(matchesA) // true
fmt.Println(matchesB) // true
fmt.Println(matchesC) // false

The Or function only returns a single result but the Many function is more complex, because you generally want to do something with the results, such as convert the runes or strings captured by the parser into another value. The parse.WithIntegerCombiner and parse.WithStringConcatCombiner functions provide some default implementations.

The examples directory contains several examples of taking the primitive parse results and returning other types such as dates and URLs.

// parse.WithIntegerCombiner concatentates the captured runes into a string,
// and parses the result to an integer.
oneToThreeNumbers := parse.Many(parse.WithIntegerCombiner,
    1, // minimum match count
    3, // maximum match count

resultA := oneToThreeNumbers(input.NewFromString("123"))
fmt.Println(resultA.Success) // true
fmt.Println(resultA.Item)    // integer value of 123

resultB := oneToThreeNumbers(input.NewFromString("1234"))
fmt.Println(resultB.Success) // true
fmt.Println(resultB.Item)    // integer value of 123

// This Many function will stop reading at the 'a'.
resultC := oneToThreeNumbers(input.NewFromString("1a234"))
fmt.Println(resultC.Success) // true
fmt.Println(resultC.Item)    // integer value of 1

// Parse letters into a string
upToThreeLetters := parse.AtMost(parse.WithStringConcatCombiner, 3, parse.Letter)
letters := upToThreeLetters(input.NewFromString("ABC1"))
// Check that we got a string back from the parser with a 'type assertion'.
resultItem, ok := letters.Item.(string)
if !ok || resultItem != "ABC" {
    t.Errorf("for 'ABC1', expected to extract 'ABC', but extracted '%v'", letters.Item)


The Scanner type combines the parser functions and Stream type to allow parsing of input files. See scanner_test.go for a working example.

stream := input.NewFromString(`<a>Example</a>`)

scanner := New(stream, xmlTokens)
var err error
for {
    item, err := scanner.Next()
    // Do something with the results based on the 
    // token's type.
    switch v := item.(type) {
        case string:
        case int:
    if err != nil {
if err != nil && err != io.EOF {