Irregular, anomalous, unnatural mean outside the sphere of what conforms to or is explainable by law, rule, custom, or principle.
Irregular implies failure to conform to a rule, a law, or a pattern, especially to one imposed for the sake of uniformity in method, practice, or conduct; thus, an irregular marriage is one that does not conform to the regulations of church or state; an irregular verse does not correspond to an accepted metrical pattern for its type; guerrilla warfare is called irregular because it does not accord with the practice of civilized nations or conventional military theory; irregular conduct may or may not be morally reprehensible, but it defies the code or standard of the community or class.
Anomalous stresses lack of conformity to what might be expected of a thing because of the class or type to which it belongs, the laws which govern its existence, or the environment in which it is found.
Sometimes it specifically implies inconsistency or a conflict of principles and sometimes it specifically means unclassifiable or indefinable.
Again, it suggests the absence of the character or of the characteristics essential to a thing of its kind or it suggests a contradiction between the professed aims or intentions of a person or institution and the conditions in which that person or institution exists or finds himself or itself at a given time.
Unnatural is the strongest of these words in its implication of censure, especially when it implies a violation of natural law or of principles accepted by all civilized men as based on reason and essential to the well-being of society. In such cases it often specifically connotes moral perversion or abnormal indifference or cruelty Sometimes the word merely means contrary to what is received as natural, either because it is not in accordance with the normal course of nature or because it is not in keeping with what one regards as normal, balanced, proper, or fitting under the circumstances.