How to identify a given string is hex color format

Note: This is strictly for validation, i.e. accepting a valid hex color. For actual parsing you won't get the individual parts out of this.

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

For ARGB:

^#(?:[0-9a-fA-F]{3,4}){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.

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.

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}$|#[0-9a-fA-F]{3}$

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