I made the following regex :
(\w{2,3})(,\s*\w{2,3})*
It mean the sentence should start with 2 or 3 letter, 2 or 3 letter as infinite. Now i should authorise the word blue and yellow.
(\w{2,3}|blue|yellow)(,\s*\w{2,3})*
It will works inly if blue and yellow are at the beginning
Is there a way to allow the exception's word after comma without repeting the word in the code ?
I'd say go with the answer given by @Toto, but if your language doesn't support recursive patterns, you could try:
^(?![, ])(?:,?\s*\b(?:\w{2,3}|blue|yellow))+$
See the online demo
^ - Start string anchor.(?![, ]) - Negative lookahead to prevent starting with a comma or space.(?: - Open 1st non-capture group.
,?\b - Match an optional comma, zero or more space characters and a word-boundary.(?: - A nested 2nd non-capture group.
\w{2,3}|blue|yellow - Lay our your options just once.) -Close 2nd non-capture group.)+ - Close 1st non capture group and match at least once.$ - End string anchor.Just be aware that \w{2,3} allows for things like __ and _1_ to be valid inputs.
If the language you are using supports recursive patterns, you can use:
^(blue|yellow|\w{2,3})(?:,\s*(?1))*$
If either blue or yellow can occur only once:
^(?:\w{2,3}\s*,\s*)*(?:blue|yellow)(?:\s*,\s*\w{2,3})*$
The pattern matches
^ Start of string(?:\w{2,3}\s*,\s*)* Optionally repeat 2-3 word chars followed by a comma(?:blue|yellow) Match either blue or yellow(?:\s*,\s*\w{2,3})* Optionally match a comma and 2-3 word chars$ End of string
