Regular expression that checks for 2 specific words

Use | for alternatives. In your case it's: (rooster|hen)

I had a similar requirement but it should only contain a particular word (from a list) and no other words should be present in the string. I had to use ^(rooster|hen)$

The expresssion to match rooster or hen as a complete word (i.e. not when they are part of a longer, different word):

\b(rooster|hen)\b

This is a safety measure to avoid false positives with partial matches.

The \b denotes a word boundary, which is the (zero-width) spot between a character in the range of "word characters" ([A-Za-z0-9_]) and any other character. In effect the above would:

• match in "A chicken is either a rooster or a hen."
• not match in "Chickens are either a roosters or hens." - but (rooster|hen) would

As a side note, to allow the plural, this would do: \b(roosters?|hens?)\b