class_docstr: A setting for entering text values limited to alphanumeric + _ values
  
  This can be used for measurement names, object names, etc.
docstr: Initializer
  
  text - the explanatory text for the setting UI
  
  value - the default / initial value
  
  first_must_be_alpha - True if the first character of the value must
                        be a letter or underbar.
docstr_tok: ["Initializer","text","-","the","explanatory","text","for","the","setting","UI","value","-","the","default","\/","initial","value","first_must_be_alpha","-","True","if","the","first","character","of","the","value","must","be","a","letter","or","underbar","."]
code: def __init__(self, text, value, *args, **kwargs):
      """Initializer
  
      text - the explanatory text for the setting UI
  
      value - the default / initial value
  
      first_must_be_alpha - True if the first character of the value must
                            be a letter or underbar.
      """
      kwargs = kwargs.copy()
      self.first_must_be_alpha = kwargs.pop(
          "first_must_be_alpha", False
      )
      super(Alphanumeric, self).__init__(text, value, *args, **kwargs)
  
imports: import re
  from .._text import Text
  from ..._validation_error import ValidationError
label_desc: Use image node_id 1 to create a new Alphanumeric object from inherited base classes: Text with example: obj = Alphanumeric(text, value)