go get github.com/alexflint/go-restructureThis package allows you to express regular expressions by defining a struct, and then capture matched sub-expressions into struct fields. Here is a very simple email address parser:
import "github.com/alexflint/go-restructure"
type EmailAddress struct {
_ struct{} `^`
User string `\w+`
_ struct{} `@`
Host string `[^@]+`
_ struct{} `$`
}
func main() {
var addr EmailAddress
restructure.Find(&addr, "joe@example.com")
fmt.Println(addr.User) // prints "joe"
fmt.Println(addr.Host) // prints "example.com"
}(Note that the above is far too simplistic to be used as a serious email address validator.)
The regular expression that was executed was the concatenation of the struct tags:
^(\w+)@([^@]+)$
The first submatch was inserted into the User field and the second into the Host field.
You may also use the regexp: tag key, but keep in mind that you must escape quotes and backslashes:
type EmailAddress struct {
_ string `regexp:"^"`
User string `regexp:"\\w+"`
_ string `regexp:"@"`
Host string `regexp:"[^@]+"`
_ string `regexp:"$"`
}Here is a slightly more sophisticated email address parser that uses nested structs:
type Hostname struct {
Domain string `\w+`
_ struct{} `\.`
TLD string `\w+`
}
type EmailAddress struct {
_ struct{} `^`
User string `[a-zA-Z0-9._%+-]+`
_ struct{} `@`
Host *Hostname
_ struct{} `$`
}
func main() {
var addr EmailAddress
success, _ := restructure.Find(&addr, "joe@example.com")
if success {
fmt.Println(addr.User) // prints "joe"
fmt.Println(addr.Host.Domain) // prints "example"
fmt.Println(addr.Host.TLD) // prints "com"
}
}Compare this to using the standard library regexp.FindStringSubmatchIndex directly:
func main() {
content := "joe@example.com"
expr := regexp.MustCompile(`^([a-zA-Z0-9._%+-]+)@((\w+)\.(\w+))$`)
indices := expr.FindStringSubmatchIndex(content)
if len(indices) > 0 {
userBegin, userEnd := indices[2], indices[3]
var user string
if userBegin != -1 && userEnd != -1 {
user = content[userBegin:userEnd]
}
domainBegin, domainEnd := indices[6], indices[7]
var domain string
if domainBegin != -1 && domainEnd != -1 {
domain = content[domainBegin:domainEnd]
}
tldBegin, tldEnd := indices[8], indices[9]
var tld string
if tldBegin != -1 && tldEnd != -1 {
tld = content[tldBegin:tldEnd]
}
fmt.Println(user) // prints "joe"
fmt.Println(domain) // prints "example"
fmt.Println(tld) // prints "com"
}
}When nesting one struct within another, you can make the nested struct optional by marking it with ?. The following example parses floating point numbers with optional sign and exponent:
// Matches "123", "1.23", "1.23e-4", "-12.3E+5", ".123"
type Float struct {
Sign *Sign `?` // sign is optional
Whole string `[0-9]*`
Period struct{} `\.?`
Frac string `[0-9]+`
Exponent *Exponent `?` // exponent is optional
}
// Matches "e+4", "E6", "e-03"
type Exponent struct {
_ struct{} `[eE]`
Sign *Sign `?` // sign is optional
Num string `[0-9]+`
}
// Matches "+" or "-"
type Sign struct {
Ch string `[+-]`
}When an optional sub-struct is not matched, it will be set to nil:
"1.23" -> {
"Sign": nil,
"Whole": "1",
"Frac": "23",
"Exponent": nil
}
"1.23e+45" -> {
"Sign": nil,
"Whole": "1",
"Frac": "23",
"Exponent": {
"Sign": {
"Ch": "+"
},
"Num": "45"
}
}The following example uses Regexp.FindAll to extract all floating point numbers from
a string, using the same Float struct as in the example above.
src := "There are 10.4 cats for every 100 dogs in the United States."
floatRegexp := restructure.MustCompile(Float{}, restructure.Options{})
var floats []Float
floatRegexp.FindAll(&floats, src, -1)To limit the number of matches set the third parameter to a positive number.
To get the begin and end position of submatches, use the restructure.Submatch struct in place of string:
Here is an example of matching python imports such as import foo as bar:
type Import struct {
_ struct{} `^import\s+`
Package restructure.Submatch `\w+`
_ struct{} `\s+as\s+`
Alias restructure.Submatch `\w+`
}
var importRegexp = restructure.MustCompile(Import{}, restructure.Options{})
func main() {
var imp Import
importRegexp.Find(&imp, "import foo as bar")
fmt.Printf("IMPORT %s (bytes %d...%d)\n", imp.Package.String(), imp.Package.Begin, imp.Package.End)
fmt.Printf(" AS %s (bytes %d...%d)\n", imp.Alias.String(), imp.Alias.Begin, imp.Alias.End)
}Output:
IMPORT foo (bytes 7...10)
AS bar (bytes 14...17)
To run a regular expression as part of a json unmarshal, just implement the JSONUnmarshaler interface. Here is an example that parses the following JSON string containing a quaternion:
{
"Var": "foo",
"Val": "1+2i+3j+4k"
}First we define the expressions for matching quaternions in the form 1+2i+3j+4k:
// Matches "1", "-12", "+12"
type RealPart struct {
Sign string `regexp:"[+-]?"`
Real string `regexp:"[0-9]+"`
}
// Matches "+123", "-1"
type SignedInt struct {
Sign string `regexp:"[+-]"`
Real string `regexp:"[0-9]+"`
}
// Matches "+12i", "-123i"
type IPart struct {
Magnitude SignedInt
_ struct{} `regexp:"i"`
}
// Matches "+12j", "-123j"
type JPart struct {
Magnitude SignedInt
_ struct{} `regexp:"j"`
}
// Matches "+12k", "-123k"
type KPart struct {
Magnitude SignedInt
_ struct{} `regexp:"k"`
}
// matches "1+2i+3j+4k", "-1+2k", "-1", etc
type Quaternion struct {
Real *RealPart
I *IPart `regexp:"?"`
J *JPart `regexp:"?"`
K *KPart `regexp:"?"`
}
// matches the quoted strings `"-1+2i"`, `"3-4i"`, `"12+34i"`, etc
type QuotedQuaternion struct {
_ struct{} `regexp:"^"`
_ struct{} `regexp:"\""`
Quaternion *Quaternion
_ struct{} `regexp:"\""`
_ struct{} `regexp:"$"`
}Next we implement UnmarshalJSON for the QuotedQuaternion type:
var quaternionRegexp = restructure.MustCompile(QuotedQuaternion{}, restructure.Options{})
func (c *QuotedQuaternion) UnmarshalJSON(b []byte) error {
if !quaternionRegexp.Find(c, string(b)) {
return fmt.Errorf("%s is not a quaternion", string(b))
}
return nil
}Now we can define a struct and unmarshal JSON into it:
type Var struct {
Name string
Value *QuotedQuaternion
}
func main() {
src := `{"name": "foo", "value": "1+2i+3j+4k"}`
var v Var
json.Unmarshal([]byte(src), &v)
}The result is:
{
"Name": "foo",
"Value": {
"Quaternion": {
"Real": {
"Sign": "",
"Real": "1"
},
"I": {
"Magnitude": {
"Sign": "+",
"Real": "2"
}
},
"J": {
"Magnitude": {
"Sign": "+",
"Real": "3"
}
},
"K": {
"Magnitude": {
"Sign": "+",
"Real": "4"
}
}
}
}
}