Filter valid hexadecimal color codes

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Old 04-20-2020
I believe valid hexadecimal color codes can have 3, 4, 6, or 8 hexadecimal digits, check out
So, your grep pattern should match all of these... You can achieve this with:

grep -E "#([[:xdigit:]]{3,4}|[[:xdigit:]]{6}|[[:xdigit:]]{8})\b" colours.txt

From man re_format: \b matches the null string at a word boundary (either the beginning or end of a word).
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pnmalias(1)						      General Commands Manual						       pnmalias(1)

pnmalias - antialias a portable anyumap. SYNOPSIS
pnmalias [-bgcolor color] [-fgcolor color] [-bonly] [-fonly] [-balias] [-falias] [-weight w] [pnmfile] DESCRIPTION
Reads a portable anymap as input, and applies anti-aliasing to background and foreground pixels. If the input file is a portable bitmap, the output anti-aliased image is promoted to a graymap, and a message is printed informing the user of the change in format. OPTIONS
-bgcolor colorb, -fgcolor colorf set the background color to colorb, and the foreground to color to colorf. Pixels with these values will be anti-aliased. by default, the background color is taken to be black, and foreground color is assumed to be white. The colors can be specified in five ways: o A name, assuming that a pointer to an X11-style color names file was compiled in. o An X11-style hexadecimal specifier: rgb:r/g/b, where r g and b are each 1- to 4-digit hexadecimal numbers. o An X11-style decimal specifier: rgbi:r/g/b, where r g and b are floating point numbers between 0 and 1. o For backwards compatibility, an old-X11-style hexadecimal number: #rgb, #rrggbb, #rrrgggbbb, or #rrrrggggbbbb. o For backwards compatibility, a triplet of numbers separated by commas: r,g,b, where r g and b are floating point numbers between 0 and 1. (This style was added before MIT came up with the similar rgbi style.) Note that even when dealing with graymaps, background and foreground colors need to be specified in the fashion described above. In this case, background and foreground pixel values are taken to be the value of the red component for the given color. -bonly, -fonly Apply anti-aliasing only to background (-bonly), or foreground (-fonly) pixels. -balias, -falias Apply anti-aliasing to all pixels surrounding background (-balias), or foreground (-falias) pixels. By default, anti-aliasing takes place only among neighboring background and foreground pixels. -weight w Use w as the central weight for the aliasing filter. W must be a real number in the range 0 < w < 1. The lower the value of w is, the "blurrier" the output image is. The default is w = 1/3. SEE ALSO
pbmtext(1), pnmsmooth(1), pnm(5) AUTHOR
Copyright (C) 1992 by Alberto Accomazzi, Smithsonian Astrophysical Observatory. 30 April 1992 pnmalias(1)

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