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Published: Sep 3, 2020 License: BSD-3-Clause Imports: 8 Imported by: 17



The licensecheck package scans source texts for known licenses. The design aims never to give a false positive. It also reports matches of known license URLs.

See the package documentation for API details.

The license scanner recognizes nearly all the licenses gathered by the SPDX project, along with a few others.

See licenses/ for license details.



Package licensecheck classifies license files and heuristically determines how well they correspond to known open source licenses.


A text (a slice of bytes) can be scanned for known licenses by calling Scan. The resulting Coverage structure describes all the matches found as well as what percentage of the file was covered by known matches.

cov := licensecheck.Scan(text)
fmt.Printf("%.1f%% of text covered by licenses:\n", cov.Percent)
for _, m := range cov.Match {
	fmt.Printf("%s at [%d:%d] IsURL=%v\n", m.Name, m.Start, m.End, m.IsURL)

The Scan function uses a built-in license set, which is the known SPDX licenses augmented with some other commonly seen licenses. (See licenses/ for details about the license set.)

A custom scanner can be created using NewScanner, passing in a set of license patterns to scan for. The license patterns are written as license regular expressions (LREs). BuiltinLicenses returns the set of license patterns used by Scan.

License Regular Expressions Each license to be recognized is specified by writing a license regular expression (LRE) for it. The pattern syntax and the matching are word-based and case-insensitive; punctuation is ignored in the pattern and in the matched text.

The valid LRE patterns are:

  • word, a single case-insensitive word
  • __N__, any sequence of up to N words
  • expr1 expr2, concatenation of two expressions
  • expr1 || expr2, alternation of two expressions
  • (( expr )), grouping
  • (( expr ))??, zero or one instances of the grouped expression
  • //** text **//, a comment ignored by the parser

To make patterns harder to misread in large texts: (( must only appear at the start of a line (possibly indented); )) and ))?? must only appear at the end of a line (with possible trailing spaces); and || must only appear inside a (( )) or (( ))?? group.

For example:

//** **//
Now is
the time for all good
((men || women || people))
to come to the aid of their __1__.

The Old Cover and Checker API

An older, less precise matcher using the names Cover, New, and Checker was removed from this package. Use v0.1.0 for the final version of that API.



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type Coverage

type Coverage struct {
	// Percent is the fraction of the total text, in normalized words, that
	// matches any valid license, expressed as a percentage across all of the
	// licenses matched.
	Percent float64

	// Match describes, in sequential order, the matches of the input text
	// across the various licenses. Typically it will be only one match long,
	// but if the input text is a concatenation of licenses it will contain
	// a match value for each element of the concatenation.
	Match []Match

Coverage describes how the text matches various licenses.

func Scan

func Scan(text []byte) Coverage

Scan computes the coverage of the text according to the license set compiled into the package.

An input text may match multiple licenses. If that happens, Match contains only disjoint matches. If multiple licenses match a particular section of the input, the best match is chosen so the returned coverage describes at most one match for each section of the input.

type License

type License struct {
	ID   string // reported license ID
	Type Type   // reported license type
	LRE  string // license regular expression (see licenses/
	URL  string // identifying URL

A License describes a single license that can be recognized. At least one of LRE or URL should be set.

func BuiltinLicenses

func BuiltinLicenses() []License

BuiltinLicenses returns the list of licenses built into the package. That is, the built-in checker is equivalent to New(BuiltinLicenses()).

type Match

type Match struct {
	ID    string // License identifier. (See licenses/
	Type  Type   // The type of the license: BSD, MIT, etc.
	Start int    // Start offset of match in text; match is at text[Start:End].
	End   int    // End offset of match in text.
	IsURL bool   // Whether match is a URL.

Match describes how a section of the input matches a license.

type Scanner

type Scanner struct {
	// contains filtered or unexported fields

A Scanner matches a set of known licenses.

func NewScanner

func NewScanner(licenses []License) (*Scanner, error)

NewScanner returns a new Scanner that recognizes the given set of licenses.

func (*Scanner) Scan

func (s *Scanner) Scan(text []byte) Coverage

Scan is like the top-level function Scan, but it uses the set of licenses in the Scanner instead of the built-in license set.

type Type

type Type uint

Type is a bit set describing the requirements imposed by a license or group of licenses.

const (
	// Unknown indicates that the license requirements are unknown.
	Unknown Type = 0

	// Unrestricted indicates that the license carries no requirements.
	// Examples: 0BSD, CC0, Unlicense.
	Unrestricted Type = 1 << iota

	// Notice indicates that the license carries notice or advertising requirements.
	// Examples: MIT, BSD, Apache 1.1, Apache 2.0

	// ShareChanges indicates that source and binary distributions must include
	// source code for any modifications made to the original code itself,
	// made available under the same license as the original.
	// Example: LGPL.

	// ShareProgram indicates that source and binary distributions must include
	// source code for the entire program using the original code library,
	// made available under the same license as the original.
	// Example: GPL.

	// ShareServer indicates that any server program making use of the source code
	// must provide a download link for the entire source code of the server.
	// Examples: AGPL, CPAL, EUPL, SSPL.

	// NonCommercial indicates that commercial use of the software is disallowed.
	// Examples: CommonsClause, CC-BY-NC-4.0, Prosperity 3.0.0.

	// Discouraged indicates that there is some legal problem with the license,
	// making it difficult to comply with or vague about what it permits.
	// Examples: Beerware, SISSL, WTFPL.

func ParseType added in v0.3.1

func ParseType(s string) (Type, error)

ParseType parses s into a Type. The string s should be of the same form returned by Type's String method.

func (Type) Merge added in v0.3.1

func (t Type) Merge(u Type) Type

Merge returns the result of merging the requirements of license types t and u.

If either is Unknown, the result is Unknown. Among the bits Unrestricted, Notice, ShareChanges, ShareProgram, ShareServer, the result will use the one that appears latest in the list and is present in either t or u. The NonCommercial and Discouraged bits are set in the result if they are set in either t or u.

func (Type) String

func (t Type) String() string

String returns the type t in string form. The form is a |-separated list of bit values (Notice, NonCommercial, and so on) with a final Type(0x###) if needed.

For example, (ShareServer|NonCommercial).String() = "ShareServer|NonCommercial".


Path Synopsis
Package match defines matching algorithms and support code for the license checker.
Package match defines matching algorithms and support code for the license checker.
Package old is an old (v0.1.0) copy of the licensecheck package, for easier comparison with the new Scan API.
Package old is an old (v0.1.0) copy of the licensecheck package, for easier comparison with the new Scan API.

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