Function Check_If_Base64(ByVal msgFile As String) As Boolean
Dim I As Long
Dim Buffer As String
Dim Car As String

Check_If_Base64 = True

Buffer = Leggi_File(msgFile)
Buffer = Replace(Buffer, vbCrLf, "")
For I = 1 To Len(Buffer)
    Car = Mid(Buffer, I, 1)
    If (Car < "A" Or Car > "Z") _
    And (Car < "a" Or Car > "z") _
    And (Car < "0" Or Car > "9") _
    And (Car <> "+" And Car <> "/" And Car <> "=") Then
        Check_If_Base64 = False
        Exit For
    End If
Next I
End Function
Function Leggi_File(PathAndFileName As String) As String
Dim FF As Integer
FF = FreeFile()
Open PathAndFileName For Binary As #FF
Leggi_File = Input(LOF(FF), #FF)
Close #FF
End Function

检查字符串的长度是否为4的倍数。然后使用这个正则表达式来确保字符串中的所有字符都是base64字符。

\A[a-zA-Z\d\/+]+={，2}\z

如果您使用的库添加换行符作为遵守每行最多76个字符规则的一种方式，请将它们替换为空字符串。

这在Python中是有效的:

def is_base64(string):
    if len(string) % 4 == 0 and re.test('^[A-Za-z0-9+\/=]+\Z', string):
        return(True)
    else:
        return(False)

对于Flutter，我测试了上面的几个注释，并将其转换为dart函数，如下所示

  static bool isBase64(dynamic value) {

    if (value.runtimeType == String){
      
      final RegExp rx = RegExp(r'^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)?$',
          multiLine: true,
          unicode: true,
      );

      final bool isBase64Valid = rx.hasMatch(value);

      if (isBase64Valid == true) {return true;}
      else {return false;}

    }

    else {return false;}

  }

当你知道原始内容的长度(例如校验和)时，这个片段可能会很有用。它检查编码的表单是否具有正确的长度。

public static boolean isValidBase64( final int initialLength, final String string ) {
  final int padding ;
  final String regexEnd ;
  switch( ( initialLength ) % 3 ) {
    case 1 :
      padding = 2 ;
      regexEnd = "==" ;
      break ;
    case 2 :
      padding = 1 ;
      regexEnd = "=" ;
      break ;
    default :
      padding = 0 ;
      regexEnd = "" ;
  }
  final int encodedLength = ( ( ( initialLength / 3 ) + ( padding > 0 ? 1 : 0 ) ) * 4 ) ;
  final String regex = "[a-zA-Z0-9/\\+]{" + ( encodedLength - padding ) + "}" + regexEnd ;
  return Pattern.compile( regex ).matcher( string ).matches() ;
}

尝试使用前面提到的正则表达式:

String regex = "^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)$";
if("TXkgdGVzdCBzdHJpbmc/".matches(regex)){
    System.out.println("it's a Base64");
}

.．.我们还可以做一个简单的验证，比如，如果它有空格，它就不能是Base64:

String myString = "Hello World";
 if(myString.contains(" ")){
   System.out.println("Not B64");
 }else{
    System.out.println("Could be B64 encoded, since it has no spaces");
 }