Pattern compile(String,int) method in Java with Examples

The compile(String, int) method of the Pattern class used to create a pattern from the regular expression with the help of flags where both expression and flags are passed as parameters to the method. The Pattern class contains a list of flags (int constants) that can be helpful to make the Pattern matching behave in certain ways. For example, The flag name CASE_INSENSITIVE is used to ignore the case of the text at the time of matching.
Syntax:

public static Pattern compile(String regex, int flags)

Parameters: This method accepts two parameters:  

• regex: This parameter represents the given regular expression compiled into a pattern.
• flag: This parameter is an integer representing Match flags, a bit mask that may include CASE_INSENSITIVE, MULTILINE, DOTALL, UNICODE_CASE, CANON_EQ, UNIX_LINES, LITERAL, UNICODE_CHARACTER_CLASS and COMMENTS.

Return Value: This method returns the pattern compiled from passed regex and flags.
Exception: This method throws following exceptions:  

• PatternSyntaxException: This exception is raised if the expression’s syntax is invalid.
• IllegalArgumentException: This exception is raised if bit values other than those corresponding to the defined match flags are set in flags.

Below programs illustrate the compile(String, int) method:
Program 1:

Java

// Java program to demonstrate
// Pattern.compile method

import java.util.regex.*;

public class GFG {
    public static void main(String[] args)
    {
        // create a REGEX String
        String REGEX = "(.*)(for)(.*)?";

        // create the string
        // in which you want to search
        String actualString
            = "code of Machine";

        // compile the regex to create pattern
        // using compile() method
        Pattern pattern = Pattern.compile(REGEX,
                           Pattern.CASE_INSENSITIVE);

        // check whether Regex string is
        // found in actualString or not
        boolean matches = pattern
                              .matcher(actualString)
                              .matches();

        System.out.println("actualString "
                           + "contains REGEX = "
                           + matches);
    }
}

Output:

actualString contains REGEX = false

Time Complexity : O(N)

Space Complexity : O(1)

Program 2:

Java

// Java program to demonstrate
// Pattern.compile method

import java.util.regex.*;

public class GFG {
    public static void main(String[] args)
    {
        // create a REGEX String
        String REGEX = ".*org.*";

        // create the string
        // in which you want to search
        String actualString
            = "geeksforgeeks.org";

        // compile the regex to create pattern
        // using compile() method
        Pattern pattern = Pattern.compile(REGEX,
                             Pattern.CASE_INSENSITIVE);

        // check whether Regex string is
        // found in actualString or not
        boolean matches = pattern
                              .matcher(actualString)
                              .matches();

        System.out.println("actualString "
                           + "contains REGEX = "
                           + matches);
    }
}

Output:

actualString contains REGEX = true

Time Complexity : O(N)

Space Complexity : O(1)

References:
https://docs.oracle.com/javase/10/docs/api/java/util/regex/Pattern.html#compile(java.lang.String, int)