Python regex for int with at least 4 digits

Question

I am just learning regex and I'm a bit confused here. I've got a string from which I want to extract an int with at least 4 digits and at most 7 digits. I tried it as follows: ``` >>> import re >>> teststring = 'abcd123efg123456' >>> re.match(r"[0-9]{4,7}$", teststring) ``` Where I was expecting 123456, unfortunately this results in nothing at all. Could anybody help me out a little bit here?

Accepted Answer

@ExplosionPills is correct, but there would still be two problems with your regex.

First, `$` matches the end of the string. I'm guessing you'd like to be able to extract an int in the middle of the string as well, e.g. `abcd123456efg789` to return `123456`. To fix that, you want this:

```
r"[0-9]{4,7}(?![0-9])"
            ^^^^^^^^^
```

The added portion is a negative lookahead assertion, meaning, "...not followed by any more numbers." Let me simplify that by the use of `\d` though:

```
r"\d{4,7}(?!\d)"
```

That's better. Now, the second problem. You have no constraint on the left side of your regex, so given a string like `abcd123efg123456789`, you'd actually match `3456789`. So, you need a negative lookbehind assertion as well:

```
r"(?<!\d)\d{4,7}(?!\d)"
```

Python regex for int with at least 4 digits

Solution

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