$12 is replaced with the 12th backreference if it exists, or with the 1st backreference followed by the literal “2” if there are less than 12 backreferences. $0 (dollar zero) inserts the entire regex match. You can use the contents of capturing parentheses in the replacement text via $1, $2, $3, etc. All parts of the string that match the regex are replaced. MyString.replaceAll("regex", "replacement") replaces all regex matches inside the string with the replacement string you specified. bc matches abc, but ^ bc $ (which is really being used here) does not. If myString is abc then myString.matches("bc") returns false. This is different from most other regex libraries, where the “quick match test” method returns true if the regex can be matched anywhere in the string. In other words: “regex” is applied as if you had written “^regex$” with start and end of string anchors. It is important to remember that String.matches() only returns true if the entire string can be matched. MyString.matches("regex") returns true or false depending whether the string can be matched entirely by the regular expression. For performance reasons, you should also not use these methods if you will be using the same regular expression often. The downside is that you cannot specify options such as “case insensitive” or “dot matches newline”. The Java String class has several methods that allow you to perform an operation using a regular expression on that string in a minimal amount of code. Java 13 allows infinite quantifiers inside lookbehind. Java 7 adds named capture and Unicode scripts. Java 6 fixes a few more bugs but doesn’t add any features. Java 5 fixes some bugs and adds support for Unicode blocks. Unless you need to support older versions of the JDK, the package is the way to go. ![]() Its quality is excellent, better than most of the 3rd party packages. ![]() I will only discuss Sun’s regex library that is now part of the JDK. Because Java lacked a regex package for so long, there are also many 3rd party regex packages available for Java. Java 4 (JDK 1.4) and later have comprehensive support for regular expressions through the standard package.
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