Added option to exclude certain characters in password generation

The code now supports the configuration parameter 'ExcludeChars', which gives users the ability to specify particular characters that should not be included in the generated passwords. This addition was fully implemented both on config.go and random.go, while the relevant option flag was also added to apg.go for user interaction.

cmd/apg/apg.go

@@ -46,6 +46,7 @@ func main() {
 	flag.BoolVar(&config.SpellPronounceable, "t", false, "")
 	flag.BoolVar(&config.CheckHIBP, "p", false, "")
 	flag.BoolVar(&showVer, "v", false, "")
+	flag.StringVar(&config.ExcludeChars, "E", "", "")
 	flag.Usage = usage
 	flag.Parse()
 

config.go

@@ -20,6 +20,9 @@ type Config struct {
 	// CheckHIBP sets a flag if the generated password has to be checked
 	// against the HIBP pwned password database
 	CheckHIBP bool
+	// ExcludeChars is a list of characters that should be excluded from
+	// generated passwords
+	ExcludeChars string
 	// FixedLength sets a fixed length for generated passwords and ignores
 	// the MinLength and MaxLength values
 	FixedLength int64

random.go

@@ -6,6 +6,8 @@ import (
 	"errors"
 	"fmt"
 	"math/big"
+	"os"
+	"regexp"
 	"strings"
 )
 
@@ -93,6 +95,16 @@ func (g *Generator) GetCharRangeFromConfig() string {
 			charRange.WriteString(CharRangeAlphaUpper)
 		}
 	}
+	if g.config.ExcludeChars != "" {
+		rex, err := regexp.Compile("[" + regexp.QuoteMeta(g.config.ExcludeChars) + "]")
+		if err == nil {
+			newRange := rex.ReplaceAllLiteralString(charRange.String(), "")
+			charRange.Reset()
+			charRange.WriteString(newRange)
+		} else {
+			_, _ = fmt.Fprintf(os.Stderr, "failed to exclude characters: %s\n", err)
+		}
+	}
 	return charRange.String()
 }