LPeg also has what it calls match-time captures (http://www.inf.puc-rio.br/~roberto/lpeg/#matchtime), which can be used to parse non-context free grammars like common TLV (tag, length, value) formats. For example, I've written a pure LPeg parser for parsing PKIX objects like X.509 certificates. Example edited code snippets with some high-level and low-level bits:

  -- returns DER object pattern that captures inner value
  local function Cobject(identifier, patt)
    local match

    if lpeg.type(patt) then
      match = function (s)
        return lpeg.match(patt * -P(1), s)
      end
    elseif type(patt) == "function" then
      match = patt
    elseif patt == nil then
      match = function (s)
        return s
      end
    else
      error(sformat("expected function, pattern or nil, got %s", type(patt)), 2)
    end

    return Cmt(identifier, function (s, pos)
      local n, pos = assert(unpacklength(s, pos))
      local s1 = s:sub(pos, pos + n - 1)
      pos = pos + n

      return (function (pos, v, ...)
        if v then
          return pos, v, ...
        else
          return false
        end
      end)(pos, match(s1))
    end)
  end

  local BIT_STRING = Cobject(P"\x03", function (s)
    local pad = s:byte(1) -- first octet is number of padding bits
    assert(pad == 0, "BIT STRING not octet aligned") -- we only support DER
    return s:sub(2)
  end)

  local IA5String = Cobject(P"\x16")

  local OID = function (oid)
    if oid then
      local s = packoid(pkix.txt2oid(oid))
      return P(sformat("\x06%s%s", packlength(#s), s)) * Cc(oid)
    else
      return Cobject(P"\x06", function (s)
        return assert(unpackoid(s))
       end)
    end
  end

  local SEQUENCE = function (patt)
    return Cobject(P"\x30", patt)
  end

  local TBSCertificate = SEQUENCE(Ct(
    Cg(Version, "version") *
    Cg(CertificateSerialNumber, "serialNumber") *
    Cg(AlgorithmIdentifier, "signature") *
    Cg(Name, "issuer") *
    Cg(Validity, "validity") *
    Cg(Name, "subject") *
    Cg(SubjectPublicKeyInfo, "subjectPublicKeyInfo") *
    Cg(UniqueIdentifier(1), "issuerUniqueID")^-1 *
    Cg(UniqueIdentifier(2), "subjectUniqueID")^-1 *
    Cg(Extensions, "extensions")^-1 *
    Cg(P(1)^1, "trash")^-1
  ))

  local Signature = BIT_STRING

  local Certificate = SEQUENCE(Ct(
    Cg(TBSCertificate, "tbsCertificate") * 
    Cg(AlgorithmIdentifier, "signatureAlgorithm") * 
    Cg(Signature, "signature")
  ))