code: def from_string(fermi_string):
      r"""Return a fermionic operator object from its string representation.
  
      The string representation is a compact format that uses the orbital index and ``'+'`` or ``'-'``
      symbols to indicate creation and annihilation operators, respectively. For instance, the string
      representation for the operator :math:`a^{\dagger}_0 a_1 a^{\dagger}_0 a_1` is
      ``'0+ 1- 0+ 1-'``. The ``'-'`` symbols can be optionally dropped such that ``'0+ 1 0+ 1'``
      represents the same operator. The format commonly used in OpenFermion to represent the same
      operator, ``'0^ 1 0^ 1'`` , is also supported.
  
      Args:
          fermi_string (str): string representation of the fermionic object
  
      Returns:
          FermiWord: the fermionic operator object
  
      **Example**
  
      >>> from_string('0+ 1- 0+ 1-')
      a⁺(0) a(1) a⁺(0) a(1)
  
      >>> from_string('0+ 1 0+ 1')
      a⁺(0) a(1) a⁺(0) a(1)
  
      >>> from_string('0^ 1 0^ 1')
      a⁺(0) a(1) a⁺(0) a(1)
  
      >>> op1 = FermiC(0) * FermiA(1) * FermiC(2) * FermiA(3)
      >>> op2 = from_string('0+ 1- 2+ 3-')
      >>> op1 == op2
      True
      """
      if fermi_string.isspace() or not fermi_string:
          return FermiWord({})
  
      fermi_string = " ".join(fermi_string.split())
  
      if not all(
          s.isdigit() or s in ["+", "-", "^", " "] for s in fermi_string
      ):
          raise ValueError(
              f"Invalid character encountered in string {fermi_string}."
          )
  
      fermi_string = re.sub(r"\^", "+", fermi_string)
  
      operators = [
          i + "-" if i[-1] not in "+-" else i
          for i in re.split(r"\s", fermi_string)
      ]
  
      return FermiWord(
          {(i, int(s[:-1])): s[-1] for i, s in enumerate(operators)}
      )
  
imports: import re
  from copy import copy
  from numbers import Number
  from numpy import ndarray
  import pennylane
label_desc: Use image node_id 1 for calling a global function with example usage: from_string(fermi_string) and returns: FermiWord, FermiWord