In English, regular nouns make their plural by adding -s (or -es, if they end in s/z sounds).
Irregular nouns make their plurals in other ways. Some examples:
Regular:
dog - dogs
cat - cats
horse - horses
bus - buses
Irregular:
man - men
mouse - mice
goose - geese
sheep - sheep
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It is irregular.
Pentagons can be regular or irregular.
It is irregular.
A pentagon can be regular or irregular.
It is an irregular polygon.