The only free and comprehensive online etymological dictionary of the Spanish language
regla f. (Noun) "rule;" "menstruation"

10th cent. From Latin regula "rod;" "rule." From regere "to rule," "to direct" (see regir).

Romance: Western Vulgar Latin: Asturian regla, Portuguese relha, Galician reixa, Catalan reixa, French règle, Italian regola ; Eastern Vulgar Latin: Romanian regulă ; Sardinian: arega
reglar (Verb) "to regulate;" "to rule"

From regla.
regresar (Verb) "to return"

16th cent. From regreso.
regresión f. (Noun) "regression"

16th cent. From Latin regressionem, accusative of regressio 'id.' Formed from regressus "return" (see regreso).
regreso m. (Noun) "return"

16th cent. From Latin regressus 'id.,' from regredi "to return," from re- and gredi "to walk" (see agredir).
reina f. (Noun) "queen"

12th cent. Originally pronounced reína, the stress was regularized in sometime in the 15th cent. From Latin regina 'id.' From Proto-Italic *reg-īna- 'id.,' formed from the adjective *reg-īno- "regal." From *rēg- "king" (see rey).

The surnames Reina, Reyna, and Reine refer to the moniker of Mary, Regina Cœli "queen of heaven," found in ancient Christian hyms.

Romance: Western Vulgar Latin: Asturian reina, Portuguese rainha, Galician raíña, Catalan reina, French reine, Italian regina ; Eastern Vulgar Latin: Romanian regină ; Sardinian: reina

Italic: Marsian regen "queen"
reino m. (Noun) "kingdom;" "reign"

12th cent. From Latin regnum 'id.' From Proto-Italic *rēg- "king" (see rey).

Romance: Western Vulgar Latin: Asturian reinu, Portuguese reino, Galician reino, Catalan regne, French règne, Italian regno ; Sardinian: regnu, arregnu
reír (Verb) "to laugh"

13th cent. From Latin redire 'id.' Of unknown origin.

Romance: Western Vulgar Latin: Asturian rir, Portuguese rir, Galician rir, Catalan riure, French rire, Italian ridere ; Eastern Vulgar Latin: Aromanian arãd, Romanian râde ; Sardinian: ridere
relación f. (Noun) "relation;" "relatoinship"

From Latin relationem, accusative of relatio 'id.' From referre "to carry back," from re- "again" (see re-) and ferre "to bring" (see -fero).

Romance: Western Vulgar Latin: Portuguese relação, French relation, Italian relazione ; Eastern Vulgar Latin: Romanian relație
reloj m. (Noun) "watch;" "clock"

Very early 15th cent. Borrowed from Old Catalan relotge 'id.' (c. 1362), earlier orrolotge "clock." From Latin horologium "clock," but originally "sundial." Borrowed from Ancient Greek ὡρολόγιον ‎(horológion) "sundial," but literally "time counter." From ὥρα ‎(hóra) "time" (see hora) and λέγειν (légein) "to count." From Proto-Indo-European *leǵ- "to collect," also whence leer.

Romance: Western Vulgar Latin: Asturian reló, Portuguese relógio, Galician reloxo, Catalan rellotge, French horloge, Italian orologio ; Eastern Vulgar Latin: Romanian orologiu ; Sardinian: arrelógiu

Italic: Paelignian lexe "you read," Marrucinian leexe 'id.'

Indo-European: Albanian: mb-ledh "to collect;" Hellenic: Ancient Greek λέγειν (légein) "to reckon"

Intriguingly, Hellenic, Albanian and Italic reflexes point to Proto-Indo-European *leǵ- while a number of words in Anatolian, Germanic and Balto-Slavic reflect another root *les- of the same meaning (cf. Hittite lešš-zi "to gather;" Gothic lisan "to collect," Old English lesan "to gather;" Lithuanian lèsti "to pick up"). Because such strong semantic concord, as well as phonological similarity, the words must be related. But how? There is no known mechanism to yield variation between *-s- and *-ǵ-.