utils/cobrautil/templates: new value type search algorithm

This patch replaces regexp with manual searching for matching bracket.
The regexp implementation is not aware of the brackets semantics and matches the last closing bracket preventing the use of <> and [] later in usage.
The current implementation finds the value type definition until the first upper case letter.

utils/cobrautil/templates/help_flags_printer.go

@@ -20,8 +20,8 @@ import (
 	"bytes"
 	"fmt"
 	"io"
-	"regexp"
 	"strings"
+	"unicode"
 
 	"github.com/mitchellh/go-wordwrap"
 	flag "github.com/spf13/pflag"
@@ -95,13 +95,12 @@ func writeFlag(out io.Writer, f *flag.Flag, envPrefix string) {
 	fmt.Fprintf(out, getFlagFormat(f), f.Shorthand, f.Name, name, def, env, usage, deprecated)
 }
 
-var valueTypeRegex = regexp.MustCompile(`^[<|\[].+[>|\]]`)
-
 func flagNameAndUsage(f *flag.Flag) (string, string) {
 	name, usage := flag.UnquoteUsage(f)
-	if vt := valueTypeRegex.FindString(usage); vt != "" {
-		name = vt
-		usage = strings.TrimSpace(usage[len(vt):])
+
+	if vt := findValueType(usage); vt > 0 {
+		name = usage[:vt]
+		usage = strings.TrimSpace(usage[vt:])
 	} else {
 		if name == "" || name == "string" {
 			name = "value"
@@ -115,6 +114,55 @@ func flagNameAndUsage(f *flag.Flag) (string, string) {
 	return name, usage
 }
 
+func findValueType(usage string) int {
+	runes := []rune(usage)
+	if len(runes) == 0 {
+		return 0
+	}
+
+	var (
+		a, b  rune
+		stack int
+	)
+	update := func(r rune) {
+		switch r {
+		case '<':
+			a, b = '<', '>'
+			stack = 1
+		case '[':
+			a, b = '[', ']'
+			stack = 1
+		}
+	}
+	update(runes[0])
+
+	if stack == 0 {
+		return 0
+	}
+
+	for i := 1; i < len(runes); i++ {
+		if stack == 0 {
+			if unicode.IsUpper(runes[i]) {
+				return i
+			}
+			update(runes[i])
+		} else {
+			switch runes[i] {
+			case a:
+				stack++
+			case b:
+				stack--
+			}
+		}
+	}
+
+	if stack > 0 {
+		panic("unbalanced brackets in usage string")
+	}
+
+	return len(runes)
+}
+
 var envReplacer = strings.NewReplacer(".", "_", "-", "_")
 
 func envName(envPrefix, flagName string) string {