Java regex hex color code

By mkyong | Updated: November 9, 2020

Viewed: 5,251 (+48 pv/w)

Tags: hex hex color code java regex regex

This article shows how to use regex to validate a hex color code.

Regex for hex color code.

  ^#([a-fA-F0-9]{6}|[a-fA-F0-9]{3})$

Regex explanation

  ^                 # start of the line
  #                 # start with a number sign `#`
  (                 # start of (group 1)
    [a-fA-F0-9]{6}  # support z-f, A-F and 0-9, with a length of 6
    |               # or
    [a-fA-F0-9]{3}  # support z-f, A-F and 0-9, with a length of 3
  )                 # end of (group 1)
  $                 # end of the line

1. Hexadecimal color code format.

In web, we can use RGB triplet or hex triplet (hex color code) to represents colors on a web page. For the hex color code, there are two formats, a standard hex triplet and shorthand hex format, both formats start with a leading number sign #.

1.1 Hex Triplet Forma
The hex triplet or standard hex color code uses six (hexadecimal)-digit form to represent color. For examples:

  #000000
  #FFFFFF
  #808080
  #8A2BE2

For #8A2BE2, the first hex 8A represents red value, the second 2B represents green value, and the third E2 represents blue value.

1.2 Shorthand Hex Format
The Shorthand hex uses three (hexadecimal)-digit form to represents a color. The idea is merely doubling each digit to become the above hex triplet format. For examples:

  #000
  #FFF
  #09C

For #000, it will become #000000. For #09C, it will become #0099CC.

Read the below CSS syntax; both represent the same color.

  p   { color: #000;    }   /* black color, hex triplet format */
  h1  { color: #000000; }   /* same color as above, shorthand hex format */

2. Java regex to validate hex color code

Below is a Java regex example to validate hex color code in standard hex triplet and shorthand formats.

package com.mkyong.regex.hex;

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

public class HexValidatorWebColor {

    private static final String HEX_WEBCOLOR_PATTERN
            = "^#([a-fA-F0-9]{6}|[a-fA-F0-9]{3})$";

    private static final Pattern pattern = Pattern.compile(HEX_WEBCOLOR_PATTERN);

    public static boolean isValid(final String colorCode) {
        Matcher matcher = pattern.matcher(colorCode);
        return matcher.matches();
    }

}

3. Tests for the regex hex color code.

Below are the unit tests to validate a list of valid and invalid hex color codes.

package com.mkyong.regex.hex;

import org.junit.jupiter.params.ParameterizedTest;
import org.junit.jupiter.params.provider.MethodSource;

import java.util.stream.Stream;

import static org.junit.jupiter.api.Assertions.assertFalse;
import static org.junit.jupiter.api.Assertions.assertTrue;

public class HexValidatorWebColorTest {

    @ParameterizedTest(name = "#{index} - Run test with web color = {0}")
    @MethodSource("validWebColorProvider")
    void test_color_regex_valid(String color) {
        assertTrue(HexValidatorWebColor.isValid(color));
    }

    @ParameterizedTest(name = "#{index} - Run test with web color = {0}")
    @MethodSource("invalidWebColorProvider")
    void test_color_regex_invalid(String color) {
        assertFalse(HexValidatorWebColor.isValid(color));
    }

    static Stream<String> validWebColorProvider() {
        return Stream.of(
                "#000000",
                "#999999",
                "#1a1a1a",
                "#1A1A1A",
                "#0f0f0f",
                "#0F0F0F",
                "#bcbcbc",
                "#BcbCbC",
                "#000",
                "#FFF",
                "#abc",
                "#def");
    }

    static Stream<String> invalidWebColorProvider() {
        return Stream.of(
                "123456",                   // must start with a #
                "#afafah",                  // support a-f only, h is not allowed
                "#123abce",                 // invalid length, must length of 3 or 6
                "#1234",                    // invalid length, must length of 3 or 6
                "#-123",                    // support 0-9 only
                " ",                        // space
                "");                        // empty
    }

}

4. Regex find hex color code from a String

Below is an example of using regex to find or extract a hex color code from a String.

package com.mkyong.regex.hex;

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

public class FindHexColorCode {

    public static void main(String[] args) {

        String str = "p { color: #000; }";

        Pattern pattern = Pattern.compile(
                "^(.*?)(#([a-fA-F0-9]{6}|[a-fA-F0-9]{3}))\\b(.*)$");

        Matcher matcher = pattern.matcher(str);
        if (matcher.matches()) {
            System.out.println(matcher.groupCount());     // 4
            System.out.println(matcher.group(0));         // p { color: #000; }
            System.out.println(matcher.group(1));         // p { color:
            System.out.println(matcher.group(2));         // #000
            System.out.println(matcher.group(3));         // 000
            System.out.println(matcher.group(4));         // ; }
        } else {
            System.out.println("no match!");
        }

    }

}

Output

4
p { color: #000; }
p { color:
#000
000
; }

In regex, the \b metacharacter means word boundry, which ensures the hex color code is not followed by any words.

Download Source Code

References

• Wikipedia – Web Color
• Wikipedia – Hexadecimal
• Regex Word Boundaries