Online Etymology Dictionary

regimental (adj.): 1650s, from regiment (n.) + -al (1). As a noun, regimentals, "dress proper to a particular regiment, military uniform," is from 1742.
regimentation (n.): 1856, noun of action from regiment (v.).
Regina: fem. proper name, from Latin, literally "queen;" related to rex (genitive regis) "king" (see regal). Cognate with Sanskrit rajni "queen," Welsh rhyain "maiden, virgin." The city in Canada was named 1882 by the then-governor general of Canada, Marquess of Lorne, in honor of Queen Victoria.
Reginald: masc. proper name, from Old High German Reginald, literally "ruling with power" (see Reynard).
region (n.): c. 1300, "tract of land of a considerable but indefinite extent," from Anglo-French regioun, Old French region "land, region, province" (12c.), from Latin regionem (nominative regio) "a district, portion of a country, territory, district; a direction, line; boundary line, limit," noun of state from past participle stem of regere "to direct, rule" (see regal). Phrase in the region of "about" (of numbers, etc.) is recorded from 1961.
regional (adj.): early 15c., from Late Latin regionalis "of or belonging to a region or province," from stem of regio (see region). Related: Regionally.
regionalism (n.): 1878, originally of Italy, "tendency toward regional loyalties" (opposed to nationalism), from regional + -ism. As "a word or phrase of local use" is from 1953.
register (n.1): late 14c., from Old French registre (13c.) and directly from Medieval Latin registrum, alteration of Late Latin regesta "list, matters recorded," noun use of Latin regesta, neuter plural of regestus, past participle of regerere "to record; retort," literally "to carry back, bring back" from re- "back" (see re-) + gerere "carry, bear" (see gest).

Also borrowed in Dutch, German, Swedish, Danish. Some senses influenced by association with Latin regere "to rule, to guide, to keep straight." Meaning in printing, "exact alignment of presswork" is from 1680s. Musical sense is from 1811, "compass or range of a voice or instrument," hence "series of tones of the same quality" (produced by a voice or instrument). Sense "device by which data is automatically recorded" is 1830, from the verb; hence Cash register (1875).
register (v.): late 14c. (transitive), "enter in a listing," from Old French registrer "note down, include" (13c.) and directly from Medieval Latin registrare, from registrum (see register (n.)). Intransitive sense, of instruments, from 1797; of persons and feelings, "make an impression," from 1901. Meaning "to enter one's name in a list" for some purpose is from 1940. Related: Registered; registering. Registered nurse attested from 1879.
register (n.2): "assistant court officer in administrative or routine function," 1530s, now chiefly U.S., alteration of registrar (q.v) due to influence of register.
registrant (n.): 1879; see register + -ant.
registrar (n.): 1670s, shortening of registrary (1540s), from Medieval Latin registrarius "one who keeps a record" (related to register (n.)). Earlier were registerer (mid-15c.), registrer (late 14c.).
registration (n.): 1560s, from Middle French registration and directly from Medieval Latin registrationem (nominative registratio) "a registering," noun of action from past participle stem of registrare (see register (v.)).
registry (n.): late 15c., "act of registering;" see register + -y (4). Meaning "book of record" is from 1620s.
regnant (adj.): "reigning, exercising authority," c. 1600, from Latin regnantem (nominative regnans) "reigning," present participle stem of regnare "to reign" (see reign). Adjective regnal (1610s) means "pertaining to a reign," especially in reference to the day or year a reign began.
regolith (n.): 1897, from Greek rhegos "rug, blanket," from PIE *reg- (3) "to dye" (see raga) + lithos "stone" (see litho-).
regress (n.): late 14c., "act of going back," from Latin regressus "a return, retreat, a going back," noun use of past participle of regredi "to go back," from re- "back" (see re-) + gradi "to step, walk" (see grade (n.)).
regress (v.): 1550s, "to return to a former state," from Latin regressus (see regress (n.)). Meaning "to move backward" is from 1823. The psychological sense of "to return to an earlier stage of life" is attested from 1926. Related: Regressed; regressing.
regression (n.): early 15c., from Latin regressionem (nominative regressio) "a going back, a return," noun of action from past participle stem of regredi (see regress (n.)).
regressive (adj.): 1630s; see regress + -ive. In reference to taxation, it is attested from 1889. Related: Regressively.
regret (v.): "to look back with distress or sorrowful longing; to grieve for on remembering," late 14c., from Old French regreter "long after, bewail, lament someone's death; ask the help of" (Modern French regretter), from re-, intensive prefix (see re-), + -greter, possibly from Frankish or some other Germanic source (compare Old English grætan "to weep;" Old Norse grata "to weep, groan"), from Proto-Germanic *gretan "weep." "Not found in other Romance languages, and variously explained" [Century Dictionary].

Related: Regretted; regretting. Replaced Old English ofþyncan, from of- "off, away," here denoting opposition, + þyncan "seem, seem fit" (as in methinks).
regret (n.): "pain or distress in the mind at something done or left undone," 1530s, from the verb, or from Middle French regret, back-formation from regreter (see regret (v.)).
regretful (adj.): 1640s, "full of regret," from regret + -ful. Regretfully, properly "with regret," incorrectly used in place of regrettably "it is to be regretted that; calling for regret" since at least 1965. "A regrettable use, prob. after HOPEFULLY adv.2" [OED].
regrettable (adj.): c. 1600, "deserving of regret," from regret + -able. Related: Regrettably.
regroup (v.): also re-group, 1838, from re- "again" + group (v.). Related: Regrouped; regrouping.
regrowth (n.): 1741, from re- + growth.
regular (adj.): 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.): 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.
regularity (n.): c. 1600, from Middle French regularite, from Medieval Latin *regularitas, from Latin regularis "having rules" (see regular (adj.)).
regularize (v.): 1620s, from regular (adj.) + -ize. Related: Regularized; regularizing.
regularly (adv.): 1520s, "at proper times," from regular + -ly (2). Meaning "in accordance with rules" is from 1560s.
regulate (v.): early 15c., "adjust by rule, control," from Late Latin regulatus, past participle of regulare "to control by rule, direct," from Latin regula "rule" (see regular). Meaning "to govern by restriction" is from 1620s. Related: Regulated; regulating.
regulation (n.): 1670s, "act of regulating; state of being reduced to order," noun of action from regulate. Meaning "rule for management" is from 1715. Related: Regulations.
regulator (n.): 1650s, agent noun in Latin form from regulate. In English history, from 1680s; in American history, from 1767, applied to local posses that kept order (or disturbed it) in rural regions. As a mechanical device or clock used to set the time of other pieces, from 1758.
Regulus (n.): bright star in constellation Leo, 1550s, Modern Latin, apparently first so-called by Copernicus, literally "little king," diminutive of rex "king;" probably a translation of Basiliskos "little king," a Hellenistic Greek name for the star, mentioned in Geminos and Ptolemy (in the "Almagest," though elsewhere in his writings it is usually "the star on the heart of Leo"); perhaps a translation of Lugal "king," said to have been the star's Babylonian name. Klein holds it to be a corruption of Arabic rijl (al-asad) "paw of the lion" (compare Rigel).
regurgitate (v.): 1640s (intransitive), 1753 (transitive), back formation from regurgitation, or else from Medieval Latin regurgitatus, past participle of regurgitare. Meaning "to vomit" first attested 1753. Related: Regurgitated; regurgitating.
regurgitation (n.): c. 1600, from Medieval Latin regurgitationem (nominative regurgitatio), noun of action from past participle stem of regurgitare "to overflow," from re- "back" (see re-) + Late Latin gurgitare "engulf, flood" (found in Latin ingurgitare "to pour in"), from gurges "whirlpool, gorge, abyss" (see gurges).
rehab: 1948 as a shortening of rehabilitation (originally of service members returning from World War II). As a verb in reference to houses, by 1975, short for rehabilitate. Related: Rehabbed; rehabbing.
rehabilitate (v.): 1570s, "to bring back to a former condition after decay or damage," back-formation from rehabilitation and in part from Medieval Latin rehabilitatus, past participle of rehabilitare. Meaning "to restore one's reputation or character in the eyes of others" is from 1847. Related: Rehabilitated; rehabilitating.
rehabilitation (n.): 1530s, from Middle French réhabilitation and directly from Medieval Latin rehabilitationem (nominative rehabilitatio) "restoration," noun of action from past participle stem of rehabilitare, from re- "again" (see re-) + habitare "make fit," from Latin habilis "easily managed, fit" (see able). Specifically of criminals, addicts, etc., from 1940.
rehash (v.): 1822, from re- "again" + hash (v.). Related: Rehashed; rehashing.
rehash (n.): 1849, from rehash (v.); "old material worked up anew," usually of literary productions.
rehearsal (n.): late 14c., "restatement, repetition of the words of another," from rehearse + -al (2), or from Old French rehearsal "a repeating." Sense in theater and music, "act of rehearsing," is from 1570s. Pre-wedding rehearsal dinner attested by 1953.
rehearse (v.): c. 1300, "to give an account of," from Anglo-French rehearser, Old French rehercier (12c.) "to go over again, repeat," literally "to rake over, turn over" (soil, ground), from re- "again" (see re-) + hercier "to drag, trail (on the ground), be dragged along the ground; rake, harrow (land); rip, tear, wound; repeat, rehearse;" from herse "a harrow" (see hearse (n.)). Meaning "to say over again, repeat what has already been said or written" is from mid-14c. in English; sense of "practice a play, part, etc." is from 1570s. Related: Rehearsed; rehearsing.
reheat (v.): 1727, from re- "again" + heat (v.). Related: Reheated; reheating.
Rehoboth: Hebrew Rehobhoth, literally "wide places" (Gen. xxvi:22).
rehouse (v.): also re-house, 1820, from re- + house (v.). Related: Rehoused; rehousing.
rehydrate (v.): 1923, from re- + hydrate (v.). Related: Rehydrated; rehydrating.
rehydration (n.): 1853, from re- + hydration.
Reich (n.): German, "kingdom, realm, state," from Old High German rihhi, related to Old English rice, from Proto-Germanic *rikja "rule" (source also of Old Norse riki, Danish rige, Old Frisian and Middle Dutch rike, Dutch rijk, Gothic reiki), from PIE *reg- (1) "move in a straight line," hence, "direct in a straight line, rule, guide" (see regal). Don Ringe, "From Proto-Indo-European to Proto-Germanic" [Oxford 2006] identifies it as a Celtic loan-word in Germanic rather than a direct evolution from PIE, based on the vowel. Used in English from 1871-1945 to refer to "the German state, Germany." Most notoriously in Third Reich (see third); there never was a First or Second in English usage.