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

This should match any #rgb, #rgba, #rrggbb, and #rrggbbaa syntax:

/^#(?:(?:[\da-f]{3}){1,2}|(?:[\da-f]{4}){1,2})$/i

break down:

^            // start of line
#            // literal pound sign, followed by
(?:          // either:
  (?:          // a non-capturing group of:
    [\da-f]{3}   // exactly 3 of: a single digit or a letter 'a'–'f'
  ){1,2}       // repeated exactly 1 or 2 times
|            // or:
  (?:          // a non-capturing group of:
    [\da-f]{4}   // exactly 4 of: a single digit or a letter 'a'–'f'
  ){1,2}       // repeated exactly 1 or 2 times
)
$            // end of line
i            // ignore case (let 'A'–'F' match 'a'–'f')

---

Notice that the above is not equivalent to this syntax, which is incorrect:

/^#(?:[\da-f]{3,4}){1,2}$/i

This would allow a group of 3 followed by a group of 4, such as #1234567, which is not a valid hex color.