Semver regex via Nix

semver.nix

let
  letter = ''[a-zA-Z]''; # unused; just from semver's spec
  positive-digit = ''[1-9]'';
  digit = ''[0-9]'';
  non-digit = ''[a-zA-Z-]'';
  identifier-character = ''[a-zA-Z0-9-]'';
  numeric-identifier = ''(0|${positive-digit}${digit}*)'';
  alphanumeric-identifier = ''(${non-digit}${identifier-character}*|${identifier-character}+${non-digit}${identifier-character}*)'';
  build-identifier = ''(${alphanumeric-identifier}|${digit}+)'';
  pre-release-identifier = ''(${alphanumeric-identifier}|${numeric-identifier})'';
  dot-separated-build-identifiers = ''${build-identifier}(\.${build-identifier})*'';
  build = dot-separated-build-identifiers;
  dot-separated-pre-release-identifiers = ''${pre-release-identifier}(\.${pre-release-identifier})*'';
  pre-release = dot-separated-pre-release-identifiers;
  patch = numeric-identifier;
  minor = numeric-identifier;
  major = numeric-identifier;
  version-core = ''${major}\.${minor}\.${patch}'';
  valid-semver = ''${version-core}(-${pre-release})?(\+${build})?'';
in
valid-semver