REGULAR

Dictionary:

- belonging to a religious order

- formed, built, arranged, or ordered according to some  established rule,law, principle, or type

- constituted, conducted, scheduled, or done in conformity with established or prescribed usages, rules, or discipline

- conforming to the normal or usual manner of inflection

Definition: mean being of the sort or kind that is expected as usual, ordinary, or average. Regular stresses conformity to a rule, standard, or pattern. Reguler, from L.L. regularis "continuing rules for guidance," from L. regula "rule," from "move in a straight line". Earliest sense was of religious orders (the opposite of secular). Extended 16c. to shapes, etc., that followed predictable or uniform patterns; sense of "normal" is from 1630s;  Origin  Middle English reguler, from Anglo-French, from Late Latin regularis regular, from Latin, of a bar, from regula rule — more at rule  First Known Use: 14th century

IRREGULAR

Dictionary:

- lacking uniformity or symmetry; uneven in shape, position, arrangement, etc 

- differing from the normal or accepted practice or routine

- not according to established standards of behaviour; unconventional 

Definition : Not regular; not conforming to a law, method, or usage recognized as the general rule; not according to common form; not conformable to nature, to the rules of moral rectitude, or to established principles; not normal; unnatural; immethodical; unsymmetrical; erratic; no straight; not uniform; as, an irregular line; an irregular figure; an irregular verse; an irregular physician; an irregular proceeding; irregular motion; irregular conduct, etc. 

Origin 

Middle English irreguler, from Anglo-French, from Late Latin irregularis not in accordance with rule, from Latin in- + regularis regular. First Known Use: 14th century

 

italian: regolare, irregolare

serbian: pravilan, nepravilan

englesh: regular, irregular

chinese: 定期, 不定期

portuguese: regulares, irregulares

russian :  регулярные, нерегулярные

arabic: العادية ، وغير النظامية

greek: τακτικές, παράτυπε

german: regelmäßige, unregelmäßige

franch: régulière, irrégulière

dutch: regelmatige, onregelmatige

             Metamorfosi, 1940, M.C. Escher

The exception that proves the rule!