java - 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^$545E

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.