var base64Rejex = /^(?:[A-Z0-9+\/]{4})*(?:[A-Z0-9+\/]{2}==|[A-Z0-9+\/]{3}=|[A-Z0-9+\/]{4})$/i;
var isBase64Valid = base64Rejex.test(base64Data); // base64Data is the base64 string

if (isBase64Valid) {
    // true if base64 formate
    console.log('It is base64');
} else {
    // false if not in base64 formate
    console.log('it is not in base64');
}

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

var base64Rejex = /^(?:[A-Z0-9+\/]{4})*(?:[A-Z0-9+\/]{2}==|[A-Z0-9+\/]{3}=|[A-Z0-9+\/]{4})$/i;
var isBase64Valid = base64Rejex.test(base64Data); // base64Data is the base64 string

if (isBase64Valid) {
    // true if base64 formate
    console.log('It is base64');
} else {
    // false if not in base64 formate
    console.log('it is not in base64');
}

如果你正在使用Java，你实际上可以使用common -codec库

import org.apache.commons.codec.binary.Base64;

String stringToBeChecked = "...";
boolean isBase64 = Base64.isArrayByteBase64(stringToBeChecked.getBytes());

[更新1]弃用通知 而不是使用

Base64.isBase64(价值);

   /**
     * Tests a given byte array to see if it contains only valid characters within the Base64 alphabet. Currently the
     * method treats whitespace as valid.
     *
     * @param arrayOctet
     *            byte array to test
     * @return {@code true} if all bytes are valid characters in the Base64 alphabet or if the byte array is empty;
     *         {@code false}, otherwise
     * @deprecated 1.5 Use {@link #isBase64(byte[])}, will be removed in 2.0.
     */
    @Deprecated
    public static boolean isArrayByteBase64(final byte[] arrayOctet) {
        return isBase64(arrayOctet);
    }

你可以:

检查长度是否为4个字符的倍数 检查每个字符都在A-Z, A-Z, 0-9， +， /集合中，除了末尾的填充为0,1或2 '='字符

如果你期望它是base64，那么你可以使用平台上可用的任何库来尝试将它解码为字节数组，如果它不是有效的base64则抛出异常。当然，这取决于你的平台。

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() ;
}