code: def _validate_linestyle(ls):
      """
      A validator for all possible line styles, the named ones *and*
      the on-off ink sequences.
      """
      if isinstance(ls, str):
          try:  # Look first for a valid named line style, like '--' or 'solid'.
              return _validate_named_linestyle(ls)
          except ValueError:
              pass
          try:
              ls = ast.literal_eval(ls)  # Parsing matplotlibrc.
          except (SyntaxError, ValueError):
              pass  # Will error with the ValueError at the end.
  
      def _is_iterable_not_string_like(x):
          # Explicitly exclude bytes/bytearrays so that they are not
          # nonsensically interpreted as sequences of numbers (codepoints).
          return np.iterable(x) and not isinstance(
              x, (str, bytes, bytearray)
          )
  
      if _is_iterable_not_string_like(ls):
          if len(ls) == 2 and _is_iterable_not_string_like(ls[1]):
              # (offset, (on, off, on, off, ...))
              offset, onoff = ls
          else:
              # For backcompat: (on, off, on, off, ...); the offset is implicit.
              offset = 0
              onoff = ls
  
          if (
              isinstance(offset, Real)
              and len(onoff) % 2 == 0
              and all(isinstance(elem, Real) for elem in onoff)
          ):
              return (offset, onoff)
  
      raise ValueError(
          f"linestyle {ls!r} is not a valid on-off ink sequence."
      )
  
imports: import ast
  from functools import lru_cache, reduce
  from numbers import Real
  import operator
  import os
  import re
  import numpy
  from matplotlib import _api, cbook
  from matplotlib.cbook import ls_mapper
  from matplotlib.colors import Colormap, is_color_like
  from matplotlib._fontconfig_pattern import parse_fontconfig_pattern
  from matplotlib._enums import JoinStyle, CapStyle
  from cycler import Cycler, cycler
label_desc: Use image node_id 28 for calling a global function with example usage: _validate_linestyle(ls) and returns: unknown, _validate_named_linestyle, offset,  onoff