什么是 pdf 文件的 base64 字符串的正则表达式？

Question

如果它是从 pdf 文件编码的，我需要验证 base64 字符串。 字符串必须是：

• 以“JVBER”开头（验证 Pdf mime 类型）
• 匹配 "^[a-zA-Z0-9+/]*={0,3}$" 并且字符串长度是 4 的倍数（验证有效的 base64 字符串）

谁能帮我将这些条件组合成一个正则表达式？

谢谢。

 public static bool HasPdfMimeType(string str)
    {
        if (!string.IsNullOrEmpty(str) && str.Length > 4)
        {
            return str.StartsWith("JVBER");
        }
        return false;
    }

 public static bool IsBase64string(string str)
    {
        if (string.IsNullOrEmpty(str))return false;

        str = str.Trim();
        return (str.Length % 4 == 0) && Regex.IsMatch(str, @"^[a-zA-Z0-9\+/]*={0,3}$", RegexOptions.None);  
    }

Answer 1

当您为此构建了几个其他库时，使用正则表达式来断言字符串长度和字符串开头字符有点不寻常，但如果您想这样做，这将起作用：

(?=^(?:.{4})*$)^JVBER[a-zA-Z0-9\\+\\/]*={0,3}$

使字符串精确为 4 的倍数是棘手的部分。

BREAKDOWN

This first portion of the regex asserts that this string's length is an exact multiple of 4.
By making a group of four characters, repeating it as many times as necessary, and anchoring
it between the beginning and end of the string, the regex is forced to accept only strings
which are multiples of 4.

(?=^(?:.{4})*$)

(?=           )    positive lookahead - make sure this is true before continuing
   ^         $     between the start and the end of the string...
    (?:    )*      ...get as many...
       .{4}        ...groupings of exactly 4 characters (any will do) as possible.


The second portion asserts the string starts with JVBER (which is %PDF encoded in Base64),
and then asserts any number of legal Base64 characters follow. The end allows between zero
and three equal signs for padding.

^JVBER[a-zA-Z0-9\+\/]*={0,3}$

^                           $    anchor between start and end of the string
 JVBER                           match "JVBER" literally
      [a-zA-Z0-9\+\/]*           match as many valid Base64 characters as needed
                      ={0,3}     match between 0 and 3 = symbols 

请注意， +和/符号已转义。 如果您在 C# 中工作，您可能想要更改此设置。

在这里试试吧！