Develop Reference

Printing Strings in Java using replaceAll

I am a beginner in Java. I don't understand how the below code is able to print all the characters in a string:-
System.out.println(yourString.replaceAll(".", "$0\n"));
I have tried reading the documentation on replaceAll and regex, still no clue.

"." is a regular expression which matches any single character. $0 in the replacement string is a placeholder for the full match of the regex. \n is a line break.
Summarized, this snippet replaces each character with itself and adds a line break after the character.

The syntax for replaceAll() method is as follows:
replaceAll(String regex, Stringreplacement) where:
regex : regular expression
replacement : replacement sequence of characters
so when you what to replace a character with \n basically every character will be printed in a different line. For example: yourString = "Hello." =>
output: Hello with every character on a different line

If the String (as you specified) is String yourString = "-"; so the result of System.out.println(yourString.replaceAll(".", "$0\n")); will be "-\n".
Actually, if you need to print all of the String characters why are you using replaceAll? Coz System.out.println(yourString); will do it perfectly.

Java replace " with \"

I am trying to replace string containing " with \" , below is the program I tried
String s="\"/test /string\"";
s = s.replaceAll("\"", "\\\"");
System.out.println(s);
But I get the same output as the string "/test /string". Why is my replace function is not working. If I do
s = s.replaceAll("\"", "\\\\\"");
then I get the output I want \"/test /string\" . Why is the former dint work , even though in code I am trying to replace " with \"

You're using String.replaceAll, which takes a regular expression as its inputs, including the replacement. As documented in Match.replaceAll():
Note that backslashes (\) and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string.
You're really just trying to do a straight replace with no regexes involved, so use String.replace instead:
s = s.replace("\"", "\\\"");

Replacing double backslashes with single backslash

I have a string "\\u003c", which belongs to UTF-8 charset. I am unable to decode it to unicode because of the presence of double backslashes. How do i get "\u003c" from "\\u003c"? I am using java.
I tried with,
myString.replace("\\\\", "\\");
but could not achieve what i wanted.
This is my code,
String myString = FileUtils.readFileToString(file);
String a = myString.replace("\\\\", "\\");
byte[] utf8 = a.getBytes();
// Convert from UTF-8 to Unicode
a = new String(utf8, "UTF-8");
System.out.println("Converted string is:"+a);
and content of the file is
\u003c

You can use String#replaceAll:
String str = "\\\\u003c";
str= str.replaceAll("\\\\\\\\", "\\\\");
System.out.println(str);
It looks weird because the first argument is a string defining a regular expression, and \ is a special character both in string literals and in regular expressions. To actually put a \ in our search string, we need to escape it (\\) in the literal. But to actually put a \ in the regular expression, we have to escape it at the regular expression level as well. So to literally get \\ in a string, we need write \\\\ in the string literal; and to get two literal \\ to the regular expression engine, we need to escape those as well, so we end up with \\\\\\\\. That is:
String Literal String Meaning to Regex
−−−−−−−−−−−−−−−−−−−−− −−−−−−−−−−−−−−−−−−−−−−−−−−− −−−−−−−−−−−−−−−−−
\ Escape the next character Would depend on next char
\\ \ Escape the next character
\\\\ \\ Literal \
\\\\\\\\ \\\\ Literal \\
In the replacement parameter, even though it's not a regex, it still treats \ and $ specially — and so we have to escape them in the replacement as well. So to get one backslash in the replacement, we need four in that string literal.

Not sure if you're still looking for a solution to your problem (since you have an accepted answer) but I will still add my answer as a possible solution to the stated problem:
String str = "\\u003c";
Matcher m = Pattern.compile("(?i)\\\\u([\\da-f]{4})").matcher(str);
if (m.find()) {
String a = String.valueOf((char) Integer.parseInt(m.group(1), 16));
System.out.printf("Unicode String is: [%s]%n", a);
}
OUTPUT:
Unicode String is: [<]
Here is online demo of the above code

Regarding the problem of "replacing double backslashes with single backslashes" or, more generally, "replacing a simple string, containing \, with a different simple string, containing \" (which is not entirely the OP problem, but part of it):
Most of the answers in this thread mention replaceAll, which is a wrong tool for the job here. The easier tool is replace, but confusingly, the OP states that replace("\\\\", "\\") doesn't work for him, that's perhaps why all answers focus on replaceAll.
Important note for people with JavaScript background:
Note that replace(CharSequence, CharSequence) in Java does replace ALL occurrences of a substring - unlike in JavaScript, where it only replaces the first one!
Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence.
On the other hand, replaceAll(String regex, String replacement) -- more docs also here -- is treating both parameters as more than regular strings:
Note that backslashes () and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string.
(this is because \ and $ can be used as backreferences to the captured regex groups, hence if you want to used them literally, you need to escape them).
In other words, both first and 2nd params of replace and replaceAll behave differently. For replace you need to double the \ in both params (standard escaping of a backslash in a string literal), whereas in replaceAll, you need to quadruple it! (standard string escape + function-specific escape)
To sum up, for simple replacements, one should stick to replace("\\\\", "\\") (it needs only one escaping, not two).
https://ideone.com/ANeMpw
System.out.println("a\\\\b\\\\c"); // "a\\b\\c"
System.out.println("a\\\\b\\\\c".replaceAll("\\\\\\\\", "\\\\")); // "a\b\c"
//System.out.println("a\\\\b\\\\c".replaceAll("\\\\\\\\", "\\")); // runtime error
System.out.println("a\\\\b\\\\c".replace("\\\\", "\\")); // "a\b\c"
https://www.ideone.com/Fj4RCO
String str = "\\\\u003c";
System.out.println(str); // "\\u003c"
System.out.println(str.replaceAll("\\\\\\\\", "\\\\")); // "\u003c"
System.out.println(str.replace("\\\\", "\\")); // "\u003c"

Another option, capture one of the two slashes and replace both slashes with the captured group:
public static void main(String args[])
{
String str = "C:\\\\";
str= str.replaceAll("(\\\\)\\\\", "$1");
System.out.println(str);
}

Try using,
myString.replaceAll("[\\\\]{2}", "\\\\");

This is for replacing the double back slash to single back slash
public static void main(String args[])
{
String str = "\\u003c";
str= str.replaceAll("\\\\", "\\\\");
System.out.println(str);
}

"\\u003c" does not 'belong to UTF-8 charset' at all. It is five UTF-8 characters: '\', '0', '0', '3', and 'c'. The real question here is why are the double backslashes there at all? Or, are they really there? and is your problem perhaps something completely different? If the String "\\u003c" is in your source code, there are no double backslashes in it at all at runtime, and whatever your problem may be, it doesn't concern decoding in the presence of double backslashes.

How do I replace all "[", "]" and double quotes in Java

I'm am having difficulty using the replaceAll method to replace square brackets and double quotes. Any ideas?
Edit:
So far I've tried:
replace("\[", "some_thing") // returns illegal escape character
replace("[[", "some_thing") // returns Unclosed character class
replace("^[", "some_thing") // returns Unclosed character class

Don't use replaceAll, use replace. The former uses regular expressions, and [] are special characters within a regex.
String replaced = input.replace("]", ""); //etc
The double quote is special in Java so you need to escape it with a single backslash ("\"").
If you want to use a regex you need to escape those characters and put them in a character class. A character class is surrounded by [] and escaping a character is done by preceding it with a backslash \. However, because a backslash is also special in Java, it also needs to be escaped, and so to give the regex engine a backslash you have to use two backslashes (\\[).
In the end it should look like this (if you were to use regex):
String replaced = input.replaceAll("[\\[\\]\"]", "");

The replaceAll method is operating against Regular Expressions. You're probably just wanting to use the "replace" method, which despite its name, does replace all occurrences.
Looking at your edit, you probably want:
someString
.replace("[", "replacement")
.replace("]", "replacement")
.replace("\"", "replacement")
or, use an appropriate regular expression, the approach I'd actually recommend if you're willing to learn regular expressions (see Mark Peter's answer for a working example).

replaceAll() takes a regex so you have to escape special characters. If you don't want all the fancy regex, use replace().
String s = "[h\"i]";
System.out.println( s.replace("[","").replace("]","").replace("\"","") );
With double quotes, you have to escape them like so: "\""

In java:
String resultString = subjectString.replaceAll("[\\[\\]\"]", "");
this will replace []" with nothing.

Alternatively, if you wished to replace ", [ and ] with different characters (instead of replacing all with empty String) you could use the replaceEachRepeatedly() method in the StringUtils class from Commons Lang.
For example,
String input = "abc\"de[fg]hi\"";
String replaced = StringUtils.replaceEachRepeatedly(input,
new String[]{"[","]","\""},
new String[]{"<open_bracket>","<close_bracket>","<double_quote>"});
System.out.println(replaced);
Prints the following:
abc<double_quote>de<open_bracket>fg<close_bracket>hi<double_quote>

String format using java

I have to make below statement as string.i am trying,but it's giving invalid character sequence.I know it is basic,But not able to do this.any help on this appreciated.
String str="_1";
'\str%' ESCAPE '\'
Output should be: '\_1%' ESCAPE '\'.
Thanks,
Chaitu

String result = "'\\" + str + "%' ESCAPE '\\'";

Inside a string, a backslash character will "escape" the character after it - which causes that character to be treated differently.
Since \ has this special meaning, if you actually want the \ character itself in the string, you need to put \\. The first backslash escapes the second, causing it to be treated as a literal \ inside the string.
Knowing this, you should be able to construct the resulting string you need. Hope this helps.

String str="_1";
String source = "'\\str%' ESCAPE '\\'";
String result = source.replaceAll("str", str);
Another way to implement string interpolation. The replaceAll function finds all occurrences of str in the source string and replaces them by the passed argument.
To encode the backslash \ in a Java string, you have to duplicate it, because a single backslash works as an escape character.
Beware that the first argument if replaceAll is actually a regular expression, so some characters have a special meaning, but for simple words it will work as expected.

String str="_1";
String output = String.format("'\\%s%%' ESCAPE '\\'",str);
System.out.println(output);//prints '\_1%' ESCAPE '\'