How to identify a given string is hex color format

Question

I\'m looking for a regular expression to validate hex colors in ASP.NET C# and
am also looking code for validation on server side.

For instance: #CCCCCC

Answer 1

Based on MSalters' answer, but preventing an incorrect match, the following works

^#(([0-9a-fA-F]{2}){3}|([0-9a-fA-F]){3})$

Or for an optional hash # symbol:

^#?(([0-9a-fA-F]{2}){3}|([0-9a-fA-F]){3})$

And without back references being generated:

^#?(?:(?:[0-9a-fA-F]{2}){3}|(?:[0-9a-fA-F]){3})$

Answer 2

Ruby

In Ruby, you have access to the \h (hexadecimal) character class. You also have to take more care of line endings, hence the \A...\z instead of the more common ^...$

/\A#(\h{3}){1,2}\z/

This will match 3 or 6 hexadecimal characters following a #. So no RGBA. It's also case-insensitive, despite not having the i flag.

Answer 3

Minor disagreement with the other solution. I'd say

^#(([0-9a-fA-F]{2}){3}|([0-9a-fA-F]){3})$

The reason is that this (correctly) captures the individual RGB components. The other expression broke #112233 in three parts, '#' 112 233. The syntax is actually '#' (RR GG BB) | (R G B)

The slight disadvantage is more backtracking is required. When parsing #CCC you don't know that the second C is the green component until you hit the end of the string; when parsing #CCCCCC you don't know that the second C is still part of the red component until you see the 4th C.

Answer 4

all answers mentioned RGB format, here is regex for ARGB format:

^#[0-9a-fA-F]{8}$|#[0-9a-fA-F]{6}$|#[0-9a-fA-F]{4}$|#[0-9a-fA-F]{3}$

Answer 5

^#(?:[0-9a-fA-F]{3}){1,2}$

Dissection:

^              anchor for start of string
#              the literal #
(              start of group
 ?:            indicate a non-capturing group that doesn't generate backreferences
 [0-9a-fA-F]   hexadecimal digit
 {3}           three times
)              end of group
{1,2}          repeat either once or twice
$              anchor for end of string

This will match an arbitrary hexadecimal color value that can be used in CSS, such as #91bf4a or #f13.

Note: No support for RGBA hex color values, though.

Answer 6

This if you want to accept named colors and rgb(a,b,c) too. The final "i" is for case insensitive.

HTML colors (#123, rgb not accepted)

/^(#[a-f0-9]{6}|black|green|silver|gray|olive|white|yellow|maroon|navy|red|blue|purple|teal|fuchsia|aqua)$/i

CSS colors (#123, rgb accepted)

/^(#[a-f0-9]{6}|#[a-f0-9]{3}|rgb *\( *[0-9]{1,3}%? *, *[0-9]{1,3}%? *, *[0-9]{1,3}%? *\)|rgba *\( *[0-9]{1,3}%? *, *[0-9]{1,3}%? *, *[0-9]{1,3}%? *, *[0-9]{1,3}%? *\)|black|green|silver|gray|olive|white|yellow|maroon|navy|red|blue|purple|teal|fuchsia|aqua)$/i