import re


def is_single_symbol(expr):
    """Returns ``True`` if the expression is a single symbol, possibly
    surrounded with white-spaces

    >>> is_single_symbol('hello')
    True

    >>> is_single_symbol('hello * world')
    False

    """
    expr = expr.strip()
    single_symbol = re.compile("^[a-zA-Z_]+[a-zA-Z_0-9]*$")
    m = single_symbol.match(expr)
    return m is not None