drive: don't use regexp package in leaf types package

Even with ts_omit_drive, the drive package is currently still imported
for some types. So it should be light. But it was depending on the
"regexp" packge, which I'd like to remove from our minimal builds.

Updates #12614

Change-Id: I5bf85d8eb15a739793723b1da11c370d3fcd2f32
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

drive/remote.go

@@ -9,7 +9,6 @@ import (
 	"bytes"
 	"errors"
 	"net/http"
-	"regexp"
 	"strings"
 )
 
@@ -21,10 +20,6 @@ var (
 	ErrInvalidShareName = errors.New("Share names may only contain the letters a-z, underscore _, parentheses (), or spaces")
 )
 
-var (
-	shareNameRegex = regexp.MustCompile(`^[a-z0-9_\(\) ]+$`)
-)
-
 // AllowShareAs reports whether sharing files as a specific user is allowed.
 func AllowShareAs() bool {
 	return !DisallowShareAs && doAllowShareAs()
@@ -125,9 +120,26 @@ func NormalizeShareName(name string) (string, error) {
 	// Trim whitespace
 	name = strings.TrimSpace(name)
 
-	if !shareNameRegex.MatchString(name) {
+	if !validShareName(name) {
 		return "", ErrInvalidShareName
 	}
 
 	return name, nil
 }
+
+func validShareName(name string) bool {
+	if name == "" {
+		return false
+	}
+	for _, r := range name {
+		if 'a' <= r && r <= 'z' || '0' <= r && r <= '9' {
+			continue
+		}
+		switch r {
+		case '_', ' ', '(', ')':
+			continue
+		}
+		return false
+	}
+	return true
+}