In python one can use `parse_expr`

to convert a string which contains valid sympy expression to an sympy expression.

I can't find how to do the same in sage. I tried `eval`

but this works sometimes and not other times. Here is an example

```
var('x')
sage: expr=sin(x)
sage: expr=str(expr)
sage: expr=eval(expr)
sin(x)
```

But not on this one

```
sage: expr=-1/2*x/(x^2 + 1)
sage: expr=str(expr)
sage: expr=eval(expr)
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
<ipython-input-16-599d1fa18625> in <module>()
----> 1 expr=eval(expr)
<string> in <module>()
/usr/lib/python2.7/site-packages/sage/structure/element.pyx in sage.structure.element.Element.__xor__ (build/cythonized/sage/structure/element.c:8986)()
951
952 def __xor__(self, right):
--> 953 raise RuntimeError("Use ** for exponentiation, not '^', which means xor\n"+\
954 "in Python, and has the wrong precedence.")
955
RuntimeError: Use ** for exponentiation, not '^', which means xor
in Python, and has the wrong precedence.
```

I do not know before hand the "type" of the expression, other than it is valid sage expression, but in a string.

I am looking for something similar to Mathematica's `ToExpression`

described in
http://reference.wolfram.com/language/ref/ToExpression.html or python `parse_expr`

I found answer here https://ask.sagemath.org/question/41135/converting-strings-into-expressions/ using something called `SR`

which I still do not understand, but it does not work for me.

```
sage: expr=integrate(x^2*(sqrt(x^2 + 1) - 2)/((x^3 - (x^2 + 1)^(3/2) - 1)*sqrt(x^2 + 1)), x)
sage: expr=str(expr)
sage: expr=SR(expr)
TypeError Traceback (most recent call last)
```

So, how would one convert string that contains arbitrary sage valid expression to sage expression?