import re


def is_indvar(expr):
    """
    An individual variable must be a single lowercase character other than 'e', 't', 'n', 's',
    followed by zero or more digits.
    
    @param expr: C{str}
    @return: C{boolean} True if expr is of the correct form 
    """
    assert isinstance(expr, str), "%s is not a string" % expr
    return re.match(r'^[a-df-mo-ru-z]\d*$', expr)