Python regex checking length of the string for two discrete values

Question

I am trying to check a string for a pattern which its length can be either 3 or 6; not the values between them.

This is the string:

color: #FfFdF8; background-color:#aef;

I want to get all sub-strings starting with # followed by a hex code, if they have the length of 3 or 6 and are not located at the beginning of the string; in this case both #FfFdF8 and #aef should be returned.

I have written this pattern:

r'^(?!#).+(#[a-fA-F0-9]{6}).*|^(?!#).+(#[a-fA-F0-9]{3}).*'

But it gave me [('#FfFdF8', '')] as the result of re.findall.

if not s.startswith('#'): results = re.findall(r'#[a-fA-F0-9]{3}(?:[a-fA-F0-9]{3})?\b', s) — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Oct 12, 2019 at 7:17
@WiktorStribiżew, it worked. Thanks. Would you please post it as an answer? — Zeinab Abbasimazar
– Zeinab Abbasimazar, Commented Oct 12, 2019 at 7:37

Wiktor Stribiżew · Accepted Answer · 2019-10-12 07:45:14Z

1

You may first check if the string starts with # and if not, extract the #... substrings:

import re
results = []
s = 'color: #FfFdF8; background-color:#aef;'
if not s.startswith('#'): 
    results = re.findall(r'#[a-fA-F0-9]{3}(?:[a-fA-F0-9]{3})?\b', s)
print(results) # => ['#FfFdF8', '#aef']

See the regex demo and the Python demo.

Regex details

# - a # char
[a-fA-F0-9]{3} - 3 hex chars
(?:[a-fA-F0-9]{3})? - an optional sequence of three hex chars
\b - a word boundary (no more hex chars to the right are allowed)

answered Oct 12, 2019 at 7:45

Wiktor Stribiżew

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