The pattern is compared to the input one character at a time, from the rightmost character in the pattern to the left. If the characters all match the pattern has been found. If a character does not match, the pattern is shifted right a distance that is the maximum of two functions, the bad character shift and the good suffix shift. This shift moves the attempted match position through the input more quickly than a naive one position at a time check.

The bad character shift is based on the character from the text that did not match. If the character does not appear in the pattern, the pattern can be shifted completely beyond the bad character. If the character does occur in the pattern, the pattern can be shifted to line the pattern up with the next occurrence of that character.

The good suffix shift is based on the idea that some subset on the right side of the pattern has matched. When a bad character is found, the pattern can be shifted right by the pattern length if the subset does not occur again in pattern, or by the amount of distance to the next occurrence of the subset in the pattern. Boyer-Moore search methods adapted from code by Amy Yu. 

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). 

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.

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). 

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).

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 CASE_INSENSITIVE and 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.

since: 1.5 -

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.)

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.

When this flag is specified then 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.

There is no embedded flag character for enabling canonical equivalence.

Specifying this flag may impose a performance penalty.

When this flag is specified then the (US-ASCII only) Predefined character classes and POSIX character classes are in conformance with Unicode Technical Standard #18: Unicode Regular Expression Annex C: Compatibility Properties.

The UNICODE_CHARACTER_CLASS mode can also be enabled via the embedded flag expression (?U).

The flag implies UNICODE_CASE, that is, it enables Unicode-aware case folding.

Specifying this flag may impose a performance penalty.

since: 1.7 -

 public static Pattern compile(String regex) 
{
        return new Pattern(regex, 0);
}

Compiles the given regular expression into a pattern.

 public static Pattern compile(String regex,
    int flags) 
{
        return new Pattern(regex, flags);
}

Compiles the given regular expression into a pattern with the given flags.

 public int flags() 
{
        return flags;
}

Returns this pattern's match flags.

 public Matcher matcher(CharSequence input) 
{
        if (!compiled) {
            synchronized(this) {
                if (!compiled)
                    compile();
            }
        }
        Matcher m = new Matcher(this, input);
        return m;
}

Creates a matcher that will match the given input against this pattern.

 public static boolean matches(String regex,
    CharSequence input) 
{
        Pattern p = Pattern.compile(regex);
        Matcher m = p.matcher(input);
        return m.matches();
}

Compiles the given regular expression and attempts to match the given input against it.

An invocation of this convenience method of the form

Pattern.matches(regex, input);

behaves in exactly the same way as the expression

Pattern.compile(regex).matcher(input).matches()

If a pattern is to be used multiple times, compiling it once and reusing it will be more efficient than invoking this method each time.

 Map<String, Integer> namedGroups() 
{
        if (namedGroups == null)
            namedGroups = new HashMap<  >(2);
        return namedGroups;
}

 public String pattern() 
{
        return pattern;
}

Returns the regular expression from which this pattern was compiled.

 public static String quote(String s) 
{
        int slashEIndex = s.indexOf("\\E");
        if (slashEIndex == -1)
            return "\\Q" + s + "\\E";
        StringBuilder sb = new StringBuilder(s.length() * 2);
        sb.append("\\Q");
        slashEIndex = 0;
        int current = 0;
        while ((slashEIndex = s.indexOf("\\E", current)) != -1) {
            sb.append(s.substring(current, slashEIndex));
            current = slashEIndex + 2;
            sb.append("\\E\\\\E\\Q");
        }
        sb.append(s.substring(current, s.length()));
        sb.append("\\E");
        return sb.toString();
}

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.

 public String[] split(CharSequence input) 
{
        return split(input, 0);
}

Splits the given input sequence around matches of this pattern.

This method works as if by invoking the two-argument split method with the given input sequence and a limit argument of zero. Trailing empty strings are therefore not included in the resulting array.

The input "boo:and:foo", for example, yields the following results with these expressions:

Regex	Result
:	{ "boo", "and", "foo" }
o	{ "b", "", ":and:f" }

 public String[] split(CharSequence input,
    int limit) 
{
        int index = 0;
        boolean matchLimited = limit  > 0;
        ArrayList< String > matchList = new ArrayList<  >();
        Matcher m = matcher(input);
        // Add segments before each match found
        while(m.find()) {
            if (!matchLimited || matchList.size() <  limit - 1) {
                String match = input.subSequence(index, m.start()).toString();
                matchList.add(match);
                index = m.end();
            } else if (matchList.size() == limit - 1) { // last one
                String match = input.subSequence(index,
                                                 input.length()).toString();
                matchList.add(match);
                index = m.end();
            }
        }
        // If no match was found, return this
        if (index == 0)
            return new String[] {input.toString()};
        // Add remaining segment
        if (!matchLimited || matchList.size() <  limit)
            matchList.add(input.subSequence(index, input.length()).toString());
        // Construct result
        int resultSize = matchList.size();
        if (limit == 0)
            while (resultSize  > 0 && matchList.get(resultSize-1).equals(""))
                resultSize--;
        String[] result = new String[resultSize];
        return matchList.subList(0, resultSize).toArray(result);
}

Splits the given input sequence around matches of this pattern.

The array returned by this method contains each substring of the input sequence that is terminated by another subsequence that matches this pattern or is terminated by the end of the input sequence. The substrings in the array are in the order in which they occur in the input. If this pattern does not match any subsequence of the input then the resulting array has just one element, namely the input sequence in string form.

The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter. If n is non-positive then the pattern will be applied as many times as possible and the array can have any length. If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded.

The input "boo:and:foo", for example, yields the following results with these parameters:

Regex	Limit	Result
:	2	{ "boo", "and:foo" }
:	5	{ "boo", "and", "foo" }
:	-2	{ "boo", "and", "foo" }
o	5	{ "b", "", ":and:f", "", "" }
o	-2	{ "b", "", ":and:f", "", "" }
o	0	{ "b", "", ":and:f" }

 public String toString() 
{
        return pattern;
}

Returns the string representation of this pattern. This is the regular expression from which this pattern was compiled.