I want to replace words if it matches with a string entirely in Python.

Suppose,

```
 st = 'Length Le'.

```

I want to replace all words 'le' with <=

Result has to be 

```
'Length <='

```

I tried this with replace() and re.sub() but it gives me 

```
'<=ngth <='

```

Is there any inbuilt function for this operation?