"Roses Are Red" is the name of a classic children's rhyming poem. It is also a class of poems inspired by that poem. It has a Roud Folk Song Index number of 19798. It is most commonly used as a love poem.
Roses are red,
Violets are blue,
Sugar is sweet,
And so are you.
The verse "Roses are red" echoes conventions traceable as far back as Edmund Spenser's epic The Faerie Queene (1590):
It was upon a Sommers shynie day,
When Titan faire his beames did display,
In a fresh fountaine, farre from all mens vew,
She bath'd her brest, the boyling heat t'allay;
She bath'd with roses red, and violets blew,
And all the sweetest flowres, that in the forrest grew.
A nursery rhyme significantly closer to the modern cliché Valentine's Day poem can be found in Gammer Gurton's Garland, a 1784 collection of English nursery rhymes:
The rose is red, the violet's blue,
The honey's sweet, and so are you.
Thou are my love and I am thine;
I drew thee to my Valentine:
The lot was cast and then I drew,
And Fortune said it shou'd be you.
Victor Hugo was likely familiar with Spenser, but may not have known the English nursery rhyme when, in 1862, he published the novel Les Misérables. Hugo was a poet as well as a novelist, and within the text of the novel are many songs. One sung by the character Fantine contains this refrain, in the 1862 Engl