java截取字符串中的指定字符的两种方法(以base64图片为例)

String base64File= "data:image/jpeg;base64,R0lGODlhHAAmAKIHAKqqqsvLy0hISObm5vf394uLiwAAA=";
int beginIndex = base64File.indexOf(":");  //开始截取的索引值
System.out.println("开始截取的索引值为：" + beginIndex);
int endIndex = base64File.indexOf(";");    //结束截取的索引值
System.out.println("结束截取的索引值为：" + endIndex);
String fileType = base64File.substring(beginIndex + 1, endIndex);  //开始截取的索引值需加 1 ，因为 substring 方法取头不取尾
System.out.println("图片类型为：" + fileType);
if(!"image/jpeg".equals(fileType)) {
    return "图片类型有误，请重传！"
}

String base64File= "data:image/jpeg;base64,R0lGODlhHAAmAKIHAKqqqsvLy0hISObm5vf394uLiwAAA=";
String fileType = base64File.substring(5, 15); 
System.out.println("图片类型为：" + fileType);

String base64File ="data:image/jpeg;base64,R0lGODlhHAAmAKIHAKqqqsvLy0hISObm5vf394uLiwAAA=";
Pattern pattern = Pattern.compile("(?<=:)[\\s\\S]*(?=;)");  //冒号和分号之间的任意字符
Matcher matcher = pattern.matcher(base64File);
if(matcher.find()) {
    System.out.println("截取的字符串为：" + matcher.group());
}