If you use one of the String methods above, the only way to specify options is to embed mode modifier into the regex. Creating a Pattern object also allows you to pass matching options as a second parameter to the pile() class factory. If you will be using the same regular expression often in your source code, you should create a Pattern object to increase performance. You should always specify Pattern.CANON_EQ to ignore differences in Unicode encodings, unless you are sure your strings contain only US ASCII characters and you want to increase performance. When working with Unicode strings, specify Pattern.UNICODE_CASE if you want to make the regex case insensitive for all characters in all languages. pile("regex", Pattern.CASE_INSENSITIVE | Pattern.DOTALL | Pattern.MULTILINE) makes the regex case insensitive for US ASCII characters, causes the dot to match line breaks and causes the start and end of string anchors to match at embedded line breaks as well. E.g.: Pattern myPattern = pile("regex") You can specify certain options as an optional second parameter. This factory returns an object of type Pattern. In Java, you compile a regular expression by using the pile() class factory. The last item in the string is the unsplit remainder of the original string. The result is that the string is split at most n-1 times. Use myString.split("regex", n) to get an array containing at most n items. The matches themselves are not included in the array. The method returns an array of strings where each element is a part of the original string between two regex matches. MyString.split("regex") splits the string at each regex match. When coding the replacement text as a literal string in your source code, remember that the backslash itself must be escaped too: "\\$". To insert a dollar sign as literal text, use \$ in the replacement text. So be careful if the replacement string is a user-specified string. If there are less than 9 backreferences, a dollar sign followed by a digit greater than the number of backreferences throws an IndexOutOfBoundsException. In the replacement text, a dollar sign not followed by a digit causes an IllegalArgumentException to be thrown. If there are 12 or more backreferences, it is not possible to insert the first backreference immediately followed by the literal “2” in the replacement text. $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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