See wikipedia's page on S-expressions.

Xgram Format is an example of an S-expression subgrammar. See e.g. DartSrc:ecfg/t/newexample.eg

The S-expression output format of the CPAN module 'Data::Stag' is another example:

 '(db
    (species_set
      (species
        (common_name "house mouse")
        (binomial "Mus musculus")
        (tax_id "10090"))
      (species
        (common_name "fruit fly")
        (binomial "Drosophila melanogaster")
        (tax_id "7227"))
      (species
        (common_name "human")
        (binomial "Homo sapiens")
        (tax_id "9606")))
    (gene_set
      (gene
        (symbol "HGNC")
        (tax_id "9606")
        (phenotype "Hemochromatosis")
        (phenotype "Porphyria variegata")
        (GO_term "iron homeostasis")
        (map
          (cytological
            (chromosome "6")
            (band "p21.3"))))
      (gene
        (symbol "Hfe")
        (synonym "MR2")
        (tax_id "10090")
        (GO_term "integral membrane protein")))
    (similarity_set
      (pair
        (symbol "HGNC")
        (symbol "Hfe"))
      (pair
        (symbol "WNT3A")
        (symbol "Wnt3a"))))