# Regular Expressions

A regular expression is a special sequence of characters that helps in matching or finding other strings or sets of strings, using a specialized syntax held in a pattern. Java has support for regular expression usage through the java.util.regex package. This topic is to introduce and help developers understand more with examples on how Regular Expressions must be used in Java.

# Using capture groups

If you need to extract a part of string from the input string, we can use capture groups of regex.

For this example, we'll start with a simple phone number regex:

\d{3}-\d{3}-\d{4}

If parentheses are added to the regex, each set of parentheses is considered a capturing group. In this case, we are using what are called numbered capture groups:

(\d{3})-(\d{3})-(\d{4})
^-----^ ^-----^ ^-----^
Group 1 Group 2 Group 3

Before we can use it in Java, we must not forget to follow the rules of Strings, escaping the backslashes, resulting in the following pattern:

"(\\d{3})-(\\d{3})-(\\d{4})"

We first need to compile the regex pattern to make a Pattern and then we need a Matcher to match our input string with the pattern:

Pattern phonePattern = Pattern.compile("(\\d{3})-(\\d{3})-(\\d{4})");
Matcher phoneMatcher = phonePattern.matcher("abcd800-555-1234wxyz");

Next, the Matcher needs to find the first subsequence that matches the regex:

phoneMatcher.find();

Now, using the group method, we can extract the data from the string:

String number = phoneMatcher.group(0); //"800-555-1234" (Group 0 is everything the regex matched)
String aCode = phoneMatcher.group(1); //"800"
String threeDigit = phoneMatcher.group(2); //"555"
String fourDigit = phoneMatcher.group(3); //"1234"

Note: Matcher.group() can be used in place of Matcher.group(0).

Java 7 introduced named capture groups. Named capture groups function the same as numbered capture groups (but with a name instead of a number), although there are slight syntax changes. Using named capture groups improves readability.

We can alter the above code to use named groups:

(?<AreaCode>\d{3})-(\d{3})-(\d{4})
^----------------^ ^-----^ ^-----^
AreaCode           Group 2 Group 3

To get the contents of "AreaCode", we can instead use:

String aCode = phoneMatcher.group("AreaCode"); //"800"

# Using regex with custom behaviour by compiling the Pattern with flags

A Pattern can be compiled with flags, if the regex is used as a literal String, use inline modifiers:

Pattern pattern = Pattern.compile("foo.", Pattern.CASE_INSENSITIVE | Pattern.DOTALL);
pattern.matcher("FOO\n").matches(); // Is true.

/* Had the regex not been compiled case insensitively and singlelined,
 * it would fail because FOO does not match /foo/ and \n (newline)
 * does not match /./.
 */

Pattern anotherPattern = Pattern.compile("(?si)foo");
anotherPattern.matcher("FOO\n").matches(); // Is true.

"foOt".replaceAll("(?si)foo", "ca"); // Returns "cat".

# Escape Characters

Generally

To use regular expression specific characters (?+| etc.) in their literal meaning they need to be escaped. In common regular expression this is done by a backslash \. However, as it has a special meaning in Java Strings, you have to use a double backslash \\.

These two examples will not work:

"???".replaceAll ("?", "!"); //java.util.regex.PatternSyntaxException
"???".replaceAll ("\?", "!"); //Invalid escape sequence

This example works

"???".replaceAll ("\\?", "!"); //"!!!"

Splitting a Pipe Delimited String

This does not return the expected result:

"a|b".split ("|"); // [a, |, b]

This returns the expected result:

"a|b".split ("\\|"); // [a, b]

Escaping backslash \

This will give an error:

"\\".matches("\\"); // PatternSyntaxException
"\\".matches("\\\"); // Syntax Error

This works:

"\\".matches("\\\\"); // true

# Not matching a given string

To match something that does not contain a given string, one can use negative lookahead:

Regex syntax: (?!string-to-not-match)

Example:

//not matching "popcorn"
String regexString = "^(?!popcorn).*$";
System.out.println("[popcorn] " + ("popcorn".matches(regexString) ? "matched!" : "nope!"));
System.out.println("[unicorn] " + ("unicorn".matches(regexString) ? "matched!" : "nope!"));

Output:

[popcorn] nope!
[unicorn] matched!

# Matching with a regex literal.

If you need to match characters that are a part of the regular expression syntax you can mark all or part of the pattern as a regex literal.

\Q marks the beginning of the regex literal. \E marks the end of the regex literal.

// the following throws a PatternSyntaxException because of the un-closed bracket
"[123".matches("[123");

// wrapping the bracket in \Q and \E allows the pattern to match as you would expect.
"[123".matches("\\Q[\\E123"); // returns true

An easier way of doing it without having to remember the \Q and \E escape sequences is to use Pattern.quote()

"[123".matches(Pattern.quote("[") + "123"); // returns true

# Matching a backslash

If you want to match a backslash in your regular expression, you'll have to escape it.

Backslash is an escape character in regular expressions. You can use '\' to refer to a single backslash in a regular expression.

However, backslash is also an escape character in Java literal strings. To make a regular expression from a string literal, you have to escape each of its backslashes. In a string literal '\\' can be used to create a regular expression with '\', which in turn can match ''.

For example, consider matching strings like "C:\dir\myfile.txt". A regular expression ([A-Za-z]):\\(.*) will match, and provide the drive letter as a capturing group. Note the doubled backslash.

To express that pattern in a Java string literal, each of the backslashes in the regular expression needs to be escaped.


   String path = "C:\\dir\\myfile.txt";
    System.out.println( "Local path: " + path ); // "C:\dir\myfile.txt"
    
    String regex = "([A-Za-z]):\\\\.*"; // Four to match one
    System.out.println("Regex:      " + regex ); // "([A-Za-z]):\\(.*)"
    
    Pattern pattern = Pattern.compile( regex );
    Matcher matcher = pattern.matcher( path );
    if ( matcher.matches()) {
        System.out.println( "This path is on drive " + matcher.group( 1 ) + ":.");
        // This path is on drive C:.
    }

If you want to match two backslashes, you'll find yourself using eight in a literal string, to represent four in the regular expression, to match two.


   String path = "\\\\myhost\\share\\myfile.txt";
    System.out.println( "UNC path: " + path ); // \\myhost\share\myfile.txt"
    
    String regex = "\\\\\\\\(.+?)\\\\(.*)"; // Eight to match two
    System.out.println("Regex:    " + regex ); // \\\\(.+?)\\(.*) 
    
    Pattern pattern = Pattern.compile( regex );
    Matcher matcher = pattern.matcher( path );
    
    if ( matcher.matches()) {
        System.out.println( "This path is on host '" + matcher.group( 1 ) + "'.");
        // This path is on host 'myhost'.
    }

# Syntax

  • Pattern patternName = Pattern.compile(regex);
  • Matcher matcherName = patternName.matcher(textToSearch);
  • matcherName.matches() //Returns true if the textToSearch exactly matches the regex
  • matcherName.find() //Searches through textToSearch for first instance of a substring matching the regex. Subsequent calls will search the remainder of the String.
  • matcherName.group(groupNum) //Returns the substring inside of a capturing group
  • matcherName.group(groupName) //Returns the substring inside of a named capturing group (Java 7+)

# Remarks

# Imports

You will need to add the following imports before you can use Regex:

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

# Pitfalls

In java, a backslash is escaped with a double backslash, so a backslash in the regex string should be inputted as a double backslash. If you need to escape a double backslash (to match a single backslash with the regex, you need to input it as a quadruple backslash.

# Important Symbols Explained

Character Description
* Match the preceding character or subexpression 0 or more times
+ Match the preceding character or subexpression 1 or more times
? Match the preceding character or subexpression 0 or 1 times

# Further reading

The regex topic contains more information about regular expressions.