What is the use of Pattern.quote method?

Question

I'm trying to understand Pattern.quote using the following code:

String pattern = Pattern.quote("1252343% 8 567 hdfg gf^$545");
System.out.println("Pattern is : "+pattern);

produces the output:

Pattern is : \Q1252343% 8 567 hdfg gf^$545\E

What are \Q and \E here? The documentation description says :

Returns a literal pattern String for the specified String.

This method produces a String that can be used to create a Pattern that would match the string s as if it were a literal pattern.

Metacharacters or escape sequences in the input sequence will be given no special meaning.

But Pattern.quote's return type is String and not a compiled Pattern object.

Why is this method required and what are some usage examples?

Answer 1

\Q means "start of literal text" (i.e. regex "open quote")
\E means "end of literal text" (i.e. regex "close quote")

Calling the Pattern.quote() method wraps the string in \Q...\E, which turns the text is into a regex literal. For example, Pattern.quote(".*") would match a dot and then an asterisk:

System.out.println("foo".matches(".*")); // true
System.out.println("foo".matches(Pattern.quote(".*"))); // false
System.out.println(".*".matches(Pattern.quote(".*"))); // true

The method's purpose is to not require the programmer to have to remember the special terms \Q and \E and to add a bit of readability to the code - regex is hard enough to read already. Compare:

someString.matches(Pattern.quote(someLiteral));
someString.matches("\\Q" + someLiteral + "\\E"));

Referring to the javadoc:

Returns a literal pattern String for the specified String.

This method produces a String that can be used to create a Pattern that would match the string s as if it were a literal pattern.

Metacharacters or escape sequences in the input sequence will be given no special meaning.