Source: semver.go in package golang.org/x/mod/semver

Package semver implements comparison of semantic version strings. In this package, semantic version strings must begin with a leading "v", as in "v1.0.0". The general form of a semantic version string accepted by this package is vMAJOR[.MINOR[.PATCH[-PRERELEASE][+BUILD]]] where square brackets indicate optional parts of the syntax; MAJOR, MINOR, and PATCH are decimal integers without extra leading zeros; PRERELEASE and BUILD are each a series of non-empty dot-separated identifiers using only alphanumeric characters and hyphens; and all-numeric PRERELEASE identifiers must not have leading zeros. This package follows Semantic Versioning 2.0.0 (see semver.org) with two exceptions. First, it requires the "v" prefix. Second, it recognizes vMAJOR and vMAJOR.MINOR (with no prerelease or build suffixes) as shorthands for vMAJOR.0.0 and vMAJOR.MINOR.0.

package semver

parsed returns the parsed form of a semantic version string.

type parsed struct {
	major      string
	minor      string
	patch      string
	short      string
	prerelease string
	build      string
	err        string
}

IsValid reports whether v is a valid semantic version string.

func IsValid(v string) bool {
	_, ok := parse(v)
	return ok
}

Canonical returns the canonical formatting of the semantic version v. It fills in any missing .MINOR or .PATCH and discards build metadata. Two semantic versions compare equal only if their canonical formattings are identical strings. The canonical invalid semantic version is the empty string.

func Canonical(v string) string {
	p, ok := parse(v)
	if !ok {
		return ""
	}
	if p.build != "" {
		return v[:len(v)-len(p.build)]
	}
	if p.short != "" {
		return v + p.short
	}
	return v
}

Major returns the major version prefix of the semantic version v. For example, Major("v2.1.0") == "v2". If v is an invalid semantic version string, Major returns the empty string.

func Major(v string) string {
	pv, ok := parse(v)
	if !ok {
		return ""
	}
	return v[:1+len(pv.major)]
}

MajorMinor returns the major.minor version prefix of the semantic version v. For example, MajorMinor("v2.1.0") == "v2.1". If v is an invalid semantic version string, MajorMinor returns the empty string.

func MajorMinor(v string) string {
	pv, ok := parse(v)
	if !ok {
		return ""
	}
	i := 1 + len(pv.major)
	if j := i + 1 + len(pv.minor); j <= len(v) && v[i] == '.' && v[i+1:j] == pv.minor {
		return v[:j]
	}
	return v[:i] + "." + pv.minor
}

Prerelease returns the prerelease suffix of the semantic version v. For example, Prerelease("v2.1.0-pre+meta") == "-pre". If v is an invalid semantic version string, Prerelease returns the empty string.

func Prerelease(v string) string {
	pv, ok := parse(v)
	if !ok {
		return ""
	}
	return pv.prerelease
}

Build returns the build suffix of the semantic version v. For example, Build("v2.1.0+meta") == "+meta". If v is an invalid semantic version string, Build returns the empty string.

func Build(v string) string {
	pv, ok := parse(v)
	if !ok {
		return ""
	}
	return pv.build
}

Compare returns an integer comparing two versions according to semantic version precedence. The result will be 0 if v == w, -1 if v < w, or +1 if v > w. An invalid semantic version string is considered less than a valid one. All invalid semantic version strings compare equal to each other.

func Compare(v, w string) int {
	pv, ok1 := parse(v)
	pw, ok2 := parse(w)
	if !ok1 && !ok2 {
		return 0
	}
	if !ok1 {
		return -1
	}
	if !ok2 {
		return +1
	}
	if c := compareInt(pv.major, pw.major); c != 0 {
		return c
	}
	if c := compareInt(pv.minor, pw.minor); c != 0 {
		return c
	}
	if c := compareInt(pv.patch, pw.patch); c != 0 {
		return c
	}
	return comparePrerelease(pv.prerelease, pw.prerelease)
}

Max canonicalizes its arguments and then returns the version string that compares greater.

func Max(v, w string) string {
	v = Canonical(v)
	w = Canonical(w)
	if Compare(v, w) > 0 {
		return v
	}
	return w
}

func parse(v string) (p parsed, ok bool) {
	if v == "" || v[0] != 'v' {
		p.err = "missing v prefix"
		return
	}
	p.major, v, ok = parseInt(v[1:])
	if !ok {
		p.err = "bad major version"
		return
	}
	if v == "" {
		p.minor = "0"
		p.patch = "0"
		p.short = ".0.0"
		return
	}
	if v[0] != '.' {
		p.err = "bad minor prefix"
		ok = false
		return
	}
	p.minor, v, ok = parseInt(v[1:])
	if !ok {
		p.err = "bad minor version"
		return
	}
	if v == "" {
		p.patch = "0"
		p.short = ".0"
		return
	}
	if v[0] != '.' {
		p.err = "bad patch prefix"
		ok = false
		return
	}
	p.patch, v, ok = parseInt(v[1:])
	if !ok {
		p.err = "bad patch version"
		return
	}
	if len(v) > 0 && v[0] == '-' {
		p.prerelease, v, ok = parsePrerelease(v)
		if !ok {
			p.err = "bad prerelease"
			return
		}
	}
	if len(v) > 0 && v[0] == '+' {
		p.build, v, ok = parseBuild(v)
		if !ok {
			p.err = "bad build"
			return
		}
	}
	if v != "" {
		p.err = "junk on end"
		ok = false
		return
	}
	ok = true
	return
}

func parseInt(v string) (t, rest string, ok bool) {
	if v == "" {
		return
	}
	if v[0] < '0' || '9' < v[0] {
		return
	}
	i := 1
	for i < len(v) && '0' <= v[i] && v[i] <= '9' {
		i++
	}
	if v[0] == '0' && i != 1 {
		return
	}
	return v[:i], v[i:], true
}

"A pre-release version MAY be denoted by appending a hyphen and a series of dot separated identifiers immediately following the patch version. Identifiers MUST comprise only ASCII alphanumerics and hyphen [0-9A-Za-z-]. Identifiers MUST NOT be empty. Numeric identifiers MUST NOT include leading zeroes."

	if v == "" || v[0] != '-' {
		return
	}
	i := 1
	start := 1
	for i < len(v) && v[i] != '+' {
		if !isIdentChar(v[i]) && v[i] != '.' {
			return
		}
		if v[i] == '.' {
			if start == i || isBadNum(v[start:i]) {
				return
			}
			start = i + 1
		}
		i++
	}
	if start == i || isBadNum(v[start:i]) {
		return
	}
	return v[:i], v[i:], true
}

func parseBuild(v string) (t, rest string, ok bool) {
	if v == "" || v[0] != '+' {
		return
	}
	i := 1
	start := 1
	for i < len(v) {
		if !isIdentChar(v[i]) && v[i] != '.' {
			return
		}
		if v[i] == '.' {
			if start == i {
				return
			}
			start = i + 1
		}
		i++
	}
	if start == i {
		return
	}
	return v[:i], v[i:], true
}

func isIdentChar(c byte) bool {
	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z' || '0' <= c && c <= '9' || c == '-'
}

func isBadNum(v string) bool {
	i := 0
	for i < len(v) && '0' <= v[i] && v[i] <= '9' {
		i++
	}
	return i == len(v) && i > 1 && v[0] == '0'
}

func isNum(v string) bool {
	i := 0
	for i < len(v) && '0' <= v[i] && v[i] <= '9' {
		i++
	}
	return i == len(v)
}

func compareInt(x, y string) int {
	if x == y {
		return 0
	}
	if len(x) < len(y) {
		return -1
	}
	if len(x) > len(y) {
		return +1
	}
	if x < y {
		return -1
	} else {
		return +1
	}
}

"When major, minor, and patch are equal, a pre-release version has lower precedence than a normal version. Example: 1.0.0-alpha < 1.0.0. Precedence for two pre-release versions with the same major, minor, and patch version MUST be determined by comparing each dot separated identifier from left to right until a difference is found as follows: identifiers consisting of only digits are compared numerically and identifiers with letters or hyphens are compared lexically in ASCII sort order. Numeric identifiers always have lower precedence than non-numeric identifiers. A larger set of pre-release fields has a higher precedence than a smaller set, if all of the preceding identifiers are equal. Example: 1.0.0-alpha < 1.0.0-alpha.1 < 1.0.0-alpha.beta < 1.0.0-beta < 1.0.0-beta.2 < 1.0.0-beta.11 < 1.0.0-rc.1 < 1.0.0."

	if x == y {
		return 0
	}
	if x == "" {
		return +1
	}
	if y == "" {
		return -1
	}
	for x != "" && y != "" {
		x = x[1:] // skip - or .
		y = y[1:] // skip - or .
		var dx, dy string
		dx, x = nextIdent(x)
		dy, y = nextIdent(y)
		if dx != dy {
			ix := isNum(dx)
			iy := isNum(dy)
			if ix != iy {
				if ix {
					return -1
				} else {
					return +1
				}
			}
			if ix {
				if len(dx) < len(dy) {
					return -1
				}
				if len(dx) > len(dy) {
					return +1
				}
			}
			if dx < dy {
				return -1
			} else {
				return +1
			}
		}
	}
	if x == "" {
		return -1
	} else {
		return +1
	}
}

func nextIdent(x string) (dx, rest string) {
	i := 0
	for i < len(x) && x[i] != '.' {
		i++
	}
	return x[:i], x[i:]