Add ParseStringRegexpv0.7.0

2 files changed, 41 insertions, 0 deletions

diff --git a/flag.go b/flag.go
index 7255802..7db895b 100644
--- a/flag.go
+++ b/flag.go
@@ -4,15 +4,22 @@
 package core
 
 import (
+	"errors"
 	"flag"
 	"fmt"
 	"os"
+	"regexp"
 	"strconv"
 	"strings"
 	"sync/atomic"
 	"time"
 )
 
+// ErrStringRegexpNoMatch is an error wrapped and returned by functions
+// created by ParseStringRegexp if the string passed did not match the
+// regular expression used.
+var ErrStringRegexpNoMatch = errors.New("string did not match regexp")
+
 // Flag works like other flag.FlagSet methods, except it is generic. The
 // passed ParseFunc will be used to parse raw arguments into a useful T
 // value. A valid *T is returned for use by the caller.
@@ -182,6 +189,18 @@ func ParseStringEnum(values ...string) ParseFunc[string] {
 	}
 }
 
+// ParseStringRegexp returns a ParseFunc that will return the string
+// passed if it matches the regular expression.
+func ParseStringRegexp(r *regexp.Regexp) ParseFunc[string] {
+	err := fmt.Errorf("%w %q", ErrStringRegexpNoMatch, r)
+	return func(s string) (string, error) {
+		if !r.MatchString(s) {
+			return "", err
+		}
+		return s, nil
+	}
+}
+
 // ParseStringerEnum returns a ParseFunc that will return the first
 // value having a string value matching the string passed.
 func ParseStringerEnum[T fmt.Stringer](values ...T) ParseFunc[T] {
diff --git a/flag_test.go b/flag_test.go
index 411174a..e5469d7 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -5,6 +5,7 @@ package core_test
 
 import (
 	"flag"
+	"regexp"
 	"strconv"
 	"testing"
 
@@ -204,6 +205,27 @@ func TestParseStringEnum(s *testing.T) {
 	})
 }
 
+func TestParseStringRegexp(s *testing.T) {
+	t := &core.T{T: s}
+	parse := core.ParseStringRegexp(regexp.MustCompile("Hello( World!)?"))
+
+	t.Run("Match", func(t *core.T) {
+		val, err := parse("Hello")
+		t.AssertErrorIs(nil, err)
+		t.AssertEqual("Hello", val)
+
+		val, err = parse("Hello World!")
+		t.AssertErrorIs(nil, err)
+		t.AssertEqual("Hello World!", val)
+	})
+
+	t.Run("NoMatch", func(t *core.T) {
+		val, err := parse("something else")
+		t.AssertErrorIs(core.ErrStringRegexpNoMatch, err)
+		t.AssertEqual("", val)
+	})
+}
+
 func TestParseStringerEnum(s *testing.T) {
 	t := &core.T{T: s, Options: cmp.Options{fakeEnumComparer}}
 	parser := core.ParseStringerEnum(fakeEnumFoo, fakeEnumBar)