add Required, Prefix, Suffix, Contains, Match validators

Broadens the built-in set with the string/presence checks that came up
most often when reaching for this package:

  - Required[T comparable]  non-zero value
  - Prefix / Suffix / Contains  strings.HasPrefix/HasSuffix/Contains
  - Match(*regexp.Regexp)  arbitrary pattern

All follow the existing shape: a constructor returning func(T) error, a
sentinel error matched with errors.Is, table-driven tests and an Example.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

match.go

@@ -0,0 +1,20 @@
+package validate
+
+import (
+	"regexp"
+)
+
+var (
+	ErrNoMatch = NewError("invalid format")
+)
+
+// Match validates that a string matches a regular expression. Compile the
+// expression once with regexp.MustCompile and reuse the returned validator.
+func Match(re *regexp.Regexp) func(string) error {
+	return func(value string) error {
+		if !re.MatchString(value) {
+			return ErrNoMatch
+		}
+		return nil
+	}
+}

match_test.go

@@ -0,0 +1,35 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+	"testing"
+)
+
+func ExampleMatch() {
+	slug := Match(regexp.MustCompile(`^[a-z0-9-]+$`))
+	fmt.Println(slug("Not A Slug"))
+	// Output: invalid format
+}
+
+func TestMatch(t *testing.T) {
+	v := Match(regexp.MustCompile(`^[a-z0-9-]+$`))
+
+	testCases := map[string]error{
+		"hello-world-123": nil,
+		"abc":             nil,
+
+		"Hello":     ErrNoMatch,
+		"has space": ErrNoMatch,
+		"":          ErrNoMatch,
+	}
+
+	for input, want := range testCases {
+		t.Run(input, func(t *testing.T) {
+			if got := v(input); !errors.Is(got, want) {
+				t.Error("got", got, "want", want)
+			}
+		})
+	}
+}

required.go

@@ -0,0 +1,21 @@
+package validate
+
+var (
+	ErrRequired = NewError("is required")
+)
+
+// Required validates that a value is not the zero value for its type: an
+// empty string, 0, a nil pointer, and so on.
+//
+// It is defined for comparable types only. For slices and maps, use
+// MinSize(1); for "non-whitespace", combine with a trimming step.
+func Required[T comparable]() func(T) error {
+	var zero T
+
+	return func(value T) error {
+		if value == zero {
+			return ErrRequired
+		}
+		return nil
+	}
+}

required_test.go

@@ -0,0 +1,46 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func ExampleRequired() {
+	notBlank := Required[string]()
+	fmt.Println(notBlank(""))
+	// Output: is required
+}
+
+func TestRequired(t *testing.T) {
+	t.Run("string", func(t *testing.T) {
+		v := Required[string]()
+		if err := v(""); !errors.Is(err, ErrRequired) {
+			t.Error("empty string should be required, got", err)
+		}
+		if err := v("x"); err != nil {
+			t.Error("non-empty string should pass, got", err)
+		}
+	})
+
+	t.Run("int", func(t *testing.T) {
+		v := Required[int]()
+		if err := v(0); !errors.Is(err, ErrRequired) {
+			t.Error("zero should be required, got", err)
+		}
+		if err := v(1); err != nil {
+			t.Error("non-zero should pass, got", err)
+		}
+	})
+
+	t.Run("pointer", func(t *testing.T) {
+		v := Required[*int]()
+		if err := v(nil); !errors.Is(err, ErrRequired) {
+			t.Error("nil pointer should be required, got", err)
+		}
+		n := 0
+		if err := v(&n); err != nil {
+			t.Error("non-nil pointer should pass, got", err)
+		}
+	})
+}

string.go

@@ -0,0 +1,41 @@
+package validate
+
+import (
+	"strings"
+)
+
+var (
+	ErrMissingPrefix    = NewError("must start with %q")
+	ErrMissingSuffix    = NewError("must end with %q")
+	ErrMissingSubstring = NewError("must contain %q")
+)
+
+// Prefix validates that a string begins with a given prefix.
+func Prefix(prefix string) func(string) error {
+	return func(value string) error {
+		if !strings.HasPrefix(value, prefix) {
+			return ErrMissingPrefix.With(prefix)
+		}
+		return nil
+	}
+}
+
+// Suffix validates that a string ends with a given suffix.
+func Suffix(suffix string) func(string) error {
+	return func(value string) error {
+		if !strings.HasSuffix(value, suffix) {
+			return ErrMissingSuffix.With(suffix)
+		}
+		return nil
+	}
+}
+
+// Contains validates that a string contains a given substring.
+func Contains(substr string) func(string) error {
+	return func(value string) error {
+		if !strings.Contains(value, substr) {
+			return ErrMissingSubstring.With(substr)
+		}
+		return nil
+	}
+}

string_test.go

@@ -0,0 +1,69 @@
+package validate
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+func ExamplePrefix() {
+	fmt.Println(Prefix("https://")("http://example.com"))
+	// Output: must start with "https://"
+}
+
+func TestPrefix(t *testing.T) {
+	testCases := map[string]error{
+		"https://example.com": nil,
+		"https://":            nil,
+
+		"http://example.com": ErrMissingPrefix.With("https://"),
+		"":                   ErrMissingPrefix.With("https://"),
+	}
+
+	v := Prefix("https://")
+	for input, want := range testCases {
+		t.Run(input, func(t *testing.T) {
+			if got := v(input); !errors.Is(got, want) {
+				t.Error("got", got, "want", want)
+			}
+		})
+	}
+}
+
+func TestSuffix(t *testing.T) {
+	testCases := map[string]error{
+		"report.pdf": nil,
+		".pdf":       nil,
+
+		"report.txt": ErrMissingSuffix.With(".pdf"),
+		"pdf":        ErrMissingSuffix.With(".pdf"),
+	}
+
+	v := Suffix(".pdf")
+	for input, want := range testCases {
+		t.Run(input, func(t *testing.T) {
+			if got := v(input); !errors.Is(got, want) {
+				t.Error("got", got, "want", want)
+			}
+		})
+	}
+}
+
+func TestContains(t *testing.T) {
+	testCases := map[string]error{
+		"a b c": nil,
+		" ":     nil,
+
+		"abc": ErrMissingSubstring.With(" "),
+		"":    ErrMissingSubstring.With(" "),
+	}
+
+	v := Contains(" ")
+	for input, want := range testCases {
+		t.Run(input, func(t *testing.T) {
+			if got := v(input); !errors.Is(got, want) {
+				t.Error("got", got, "want", want)
+			}
+		})
+	}
+}