regularyoudaoicibaDictYouDict[regular 词源字典]
regular: [14] Regular ‘according to a rule’ is the most instantly recognizable English descendant of Latin rēgula ‘rule’ (others include rail ‘bar’ and rule). It goes back ultimately to the same Indo-European base as produced Latin regere ‘rule’ (source of English rector, regent, etc) and rēx ‘king’ (source of English regal, royal, etc). From it was derived the late Latin verb rēgulāre, which has given English regulate [17], and may also lie behind rile ‘annoy’ [19], a variant of an earlier roil which was possibly imported via Old French ruiler ‘mix mortar’.
=> rector, regent, regulate, rile, rule[regular etymology, regular origin, 英语词源]
regular (adj.)youdaoicibaDictYouDict
late 14c., from Old French reguler "ecclesiastical" (Modern French régulier), from Late Latin regularis "containing rules for guidance," from Latin regula "rule," from PIE *reg- "move in a straight line" (see regal).

Earliest sense was of religious orders (the opposite of secular). Extended from late 16c. to shapes, etc., that followed predictable or uniform patterns; sense of "normal" is from 1630s; meaning "real, genuine" is from 1821. Old English borrowed Latin regula and nativized it as regol "rule, regulation, canon, law, standard, pattern;" hence regolsticca "ruler" (instrument); regollic (adj.) "canonical, regular."
regular (n.)youdaoicibaDictYouDict
c. 1400, "member of a religious order," from regular (adj.). Sense of "soldier of a standing army" is from 1756. Meaning "regular customer" is from 1852; meaning "leaded gasoline" is from 1978.