The Pattern Class

Creating a Pattern with Flags

The Pattern class defines an alternate compile method that accepts a set of flags affecting the way the pattern is matched. The flags parameter is a bit mask that may include any of the following public static fields:

• Pattern.CANON_EQ Enables canonical equivalence. When this flag is specified, two characters will be considered to match if, and only if, their full canonical decompositions match. The expression "a\u030A", for example, will match the string "\u00E5" when this flag is specified. By default, matching does not take canonical equivalence into account. Specifying this flag may impose a performance penalty.
• Pattern.CASE_INSENSITIVE Enables case-insensitive matching. By default, case-insensitive matching assumes that only characters in the US-ASCII charset are being matched. Unicode-aware case-insensitive matching can be enabled by specifying the UNICODE_CASE flag in conjunction with this flag. Case-insensitive matching can also be enabled via the embedded flag expression (?i). Specifying this flag may impose a slight performance penalty.
• Pattern.COMMENTS Permits whitespace and comments in the pattern. In this mode, whitespace is ignored, and embedded comments starting with # are ignored until the end of a line. Comments mode can also be enabled via the embedded flag expression (?x).
• Pattern.DOTALL Enables dotall mode. In dotall mode, the expression . matches any character, including a line terminator. By default this expression does not match line terminators. Dotall mode can also be enabled via the embedded flag expression (?s). (The s is a mnemonic for "single-line" mode, which is what this is called in Perl.)
• Pattern.LITERAL Enables literal parsing of the pattern. When this flag is specified then the input string that specifies the pattern is treated as a sequence of literal characters. Metacharacters or escape sequences in the input sequence will be given no special meaning. The flags Pattern.CASE_INSENSITIVE and Pattern.UNICODE_CASE retain their impact on matching when used in conjunction with this flag. The other flags become superfluous. There is no embedded flag character for enabling literal parsing.
• Pattern.MULTILINE Enables multiline mode. In multiline mode the expressions ^ and $ match just after or just before, respectively, a line terminator or the end of the input sequence. By default these expressions only match at the beginning and the end of the entire input sequence. Multiline mode can also be enabled via the embedded flag expression (?m).
• Pattern.UNICODE_CASE Enables Unicode-aware case folding. When this flag is specified then case-insensitive matching, when enabled by the CASE_INSENSITIVE flag, is done in a manner consistent with the Unicode Standard. By default, case-insensitive matching assumes that only characters in the US-ASCII charset are being matched. Unicode-aware case folding can also be enabled via the embedded flag expression (?u). Specifying this flag may impose a performance penalty.
• Pattern.UNIX_LINES Enables UNIX lines mode. In this mode, only the \n line terminator is recognized in the behavior of ., ^, and $. UNIX lines mode can also be enabled via the embedded flag expression (?d).

In the following steps we will modify the test harness, RegexTestHarness.java to create a pattern with case-insensitive matching.

First, modify the code to invoke the alternate version of compile:

Pattern pattern = 
Pattern.compile(
    console.readLine("%nEnter your regex: "),
    Pattern.CASE_INSENSITIVE);

Then compile and run the test harness to get the following results:

Enter your regex: dog
Enter input string to search: DoGDOg
I found the text "DoG" starting at index 0 and ending at index 3.
I found the text "DOg" starting at index 3 and ending at index 6.

As you can see, the string literal "dog" matches both occurences, regardless of case. To compile a pattern with multiple flags, separate the flags to be included using the bitwise OR operator |. For clarity, the following code samples hardcode the regular expression instead of reading it from the Console:

pattern = Pattern.compile("[az]$", Pattern.MULTILINE | Pattern.UNIX_LINES);

You could also specify an int variable instead:

int flags = Pattern.CASE_INSENSITIVE | Pattern.UNICODE_CASE;
Pattern pattern = Pattern.compile("aa", flags);

Embedded Flag Expressions

It is also possible to enable various flags using embedded flag expressions. Embedded flag expressions are an alternative to the two-argument version of compile, and are specified in the regular expression itself. The following example uses the original test harness, RegexTestHarness.java with the embedded flag expression (?i) to enable case-insensitive matching.

Enter your regex: (?i)foo
Enter input string to search: FOOfooFoOfoO
I found the text "FOO" starting at index 0 and ending at index 3.
I found the text "foo" starting at index 3 and ending at index 6.
I found the text "FoO" starting at index 6 and ending at index 9.
I found the text "foO" starting at index 9 and ending at index 12.

Once again, all matches succeed regardless of case.

The embedded flag expressions that correspond to Pattern's publicly accessible fields are presented in the following table:

Using the Match Static Method

The Pattern class defines a convenient matches method that allows you to quickly check if a pattern is present in a given input string. As with all public static methods, you should invoke matches by its class name, such as Pattern.matches("\\d","1");. In this example, the method returns true, because the digit "1" matches the regular expression \d.

Using the Split Method

The split() method is a great tool for gathering the text that lies on either side of the pattern that's been matched. As shown below, the split() method could extract the words "one two three four five" from the string "one:two:three:four:five":

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class SplitDemo {

    private static final String REGEX = ":";
    private static final String INPUT =
        "one:two:three:four:five";
    
    public static void main(String[] args) {
        Pattern p = Pattern.compile(REGEX);
        String[] items = p.split(INPUT);
        for(String s : items) {
            System.out.println(s);
        }
    }
}

Running this code produces the following result:

one
two
three
four
five

For simplicity, you have matched a string literal, the colon (:) instead of a complex regular expression. Since we are still using Pattern and Matcher objects, you can use split() to get the text that falls on either side of any regular expression. Here is the same example, modified to split on digits instead:

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class SplitDemo2 {

    private static final String REGEX = "\\d";
    private static final String INPUT =
        "one9two4three7four1five";

    public static void main(String[] args) {
        Pattern p = Pattern.compile(REGEX);
        String[] items = p.split(INPUT);
        for(String s : items) {
            System.out.println(s);
        }
    }
}

Running this code produces the same result:

one
two
three
four
five

Other Utility Methods

You may find the following methods to be of some use as well:

• public static String quote(String s) 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 String s as if it were a literal pattern. Metacharacters or escape sequences in the input sequence will be given no special meaning.
• public String toString() Returns the String representation of this pattern. This is the regular expression from which this pattern was compiled.

Pattern Method Equivalents in String

Regular expression support also exists in java.lang.String through several methods that mimic the behavior of java.util.regex.Pattern. For convenience, key excerpts from their API are presented below.

• public boolean matches(String regex): Tells whether or not this string matches the given regular expression. An invocation of this method of the form str.matches(regex) yields exactly the same result as the expression Pattern.matches(regex, str).
• public String[] split(String regex, int limit): Splits this string around matches of the given regular expression. An invocation of this method of the form str.split(regex, n) yields the same result as the expression Pattern.compile(regex).split(str, n)
• public String[] split(String regex): Splits this string around matches of the given regular expression. This method works the same as if you invoked the two-argument split method with the given expression and a limit argument of zero. Trailing empty strings are not included in the resulting array.

There is also a replace() method, that replaces one CharSequence with another:

• public String replace(CharSequence target,CharSequence replacement): Replaces each substring of this string that matches the literal target sequence with the specified literal replacement sequence. The replacement proceeds from the beginning of the string to the end, for example, replacing "aa" with "b" in the string "aaa" will result in "ba" rather than "ab".