java is the most fallible special characters

Background problem

The following java can say exactly after the implementation of what will print out?

        System.out.println(
                String.class.getName()+ ".class");
        System.out.println(
                String.class.getName().
                replaceAll(".","/") + ".class");

I believe for the first row, most people do not make mistakes, print

java.lang.String.class

We want to use / to split the class package, look forward to the results print

java/lang/String/class

The results of the real return is like this:

////////////////.class

Why is this so

 the reason

　　The problem is that String.replaceAll received a regular expression as its first argument, and

Non-acceptance of a literal sequence of characters . (Regular expressions has been added to the Java Platform 1.4

Version. ) Regular expression. "" Matches any single character, and therefore, the name of each class

Characters have been replaced with a slash, and produce the output we see.

Solutions

Method 1: Use the escape character

        System.out.println(
                String.class.getName().
                replaceAll("\\.","/") + ".class");    

 

Print results

java/lang/String.class

Is not it a bit do not understand, why are there two?

The first "\" represents the reference (Quotation regular expressions), second code "\" escape

Quotation
 \	Nothing, but quotes the following character
 \Q	Nothing, but quotes all characters until \E
 \E	Nothing, but ends quoting started by \Q

Second way to use Quotation

        System.out.println(
                String.class.getName().
                replaceAll("\\Q.\\E","/") + ".class");    

The result is

java/lang/String.class

You can also use

        System.out.println(
                String.class.getName().
                replaceAll(Pattern.quote("."),"/") + ".class");    

Its internal implementation is to use Quotation

 /**
 * Returns a literal pattern <code>String</code> for the specified
 * <code>String</code>.
 *
 * <p>This method produces a <code>String</code> that can be used to
 * create a <code>Pattern</code> that would match the string
 * <code>s</code> as if it were a literal pattern.</p> Metacharacters
 * or escape sequences in the input sequence will be given no special
 * meaning.
 *
 * @param s The string to be literalized
 * @return A literal string replacement
 * @since 1.5
 */
 public static String quote(String s) {
 int slashEIndex = s.indexOf("\\E");
 if (slashEIndex == -1)
 return "\\Q" + s + "\\E";
 StringBuilder sb = new StringBuilder(s.length() * 2);
 sb.append("\\Q");
 slashEIndex = 0;
 int current = 0;
 while ((slashEIndex = s.indexOf("\\E", current)) != -1) {
 sb.append(s.substring(current, slashEIndex));
 current = slashEIndex + 2;
 sb.append("\\E\\\\E\\Q");
 }
 sb.append(s.substring(current, s.length()));
 sb.append("\\E");
 return sb.toString();
 }

 

Common special characters are:

EscapeSequence:
\ b (backspace BS, Unicode \\u0008)
\ t (horizontal tab HT, Unicode \\u0009)
\ n (linefeed LF, Unicode \\u000a)
\ f (form feed FF, Unicode \\u000c)
\ r (carriage return CR, Unicode \\u000d)
\ " (double quote ", Unicode \\u0022)
\ ' (single quote ', Unicode \\u0027)
\ \ (backslash \, Unicode \\u005c)
OctalEscape (octal value, Unicode \\u0000 to \\u00ff)

and also

Twelve tokens, formed from ASCII characters, are the separators (punctuators).

( ) { } [ ] ; , . ... @ ::

It may also be determined using the following method

import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
public class CheckSpecialCharacterString {
 
 /**
 * Check whether the each character of String is special character or not using java
 * @author www.instanceofjava.com
 */
 
public static void main(String[] args) {
String Str="Java String interview questions*$%";
 
String specialCharacters=" !#$%&'()*+,-./:;<=>?@[]^_`{|}";
 
for (int i = 0; i < Str.length(); i++) {
 
 if (specialCharacters.contains(Character.toString(Str.charAt(i))))
 {
 
 System.out.println(Str.charAt(i)+": is a special character");
 } 
 }
 
}
 
}

 

More detailed information can refer to the official documentation [3]

References:

[1] java doubts

【2】https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html

【3】https://docs.oracle.com/javase/specs/jls/se12/html/jls-3.html#jls-3.10.6

【4】http://www.instanceofjava.com/2017/05/how-to-check-if-character-is-special.html