Regular Expressions and Building Regexes in Python (Summary)

Thank you for the good course. Everything well to the point. Will help me in my work.

The instructor definitely has a great pedagogical skill! Thank you.

This was a great course and I learned a lot. I used it to complete some assignments in another regex module on Coursera that didn’t provide the instruction I needed to fully understand the concepts. I would have liked a few examples of re.finditer() and when to use that. Otherwise, it was outstanding. Thanks!!

Hi datagirl-89,

I’m glad you found the course useful.

Both re.findall() and re.finditer() are for finding multiple matches inside of some text. The re.findall() method returns just the matching strings, whereas re.finditer() returns match objects (this is actually a bit inconsistent and confusing).

The other difference is re.findall() returns a list, whereas re.finditer() returns an iterator. If you haven’t come across iterators before, they’re used similarly to lists, but they calculate results on the fly. Lists are precalculated and take up the amount of memory of everything in the list. Iterators can be used the same way you use lists, but they don’t precalulate everything, they calculate the item when you ask for it.

More on iterators here:

realpython.com/python-for-loop/#iterators

As for a quick example:

>>> import re
>>> text = "She sells sea shells by the sea shore"
>>> re.findall(r'se', text)
['se', 'se', 'se']
>>> re.finditer(r'se', text)
<callable_iterator object at 0x7f944ff29880>
>>> for match in re.finditer(r'se', text):
...     print(match)
...
<re.Match object; span=(4, 6), match='se'>
<re.Match object; span=(10, 12), match='se'>
<re.Match object; span=(28, 30), match='se'>

Generally you want to prefer iterators over lists whenever dealing with large amounts of data to avoid using up too much memory. In this particular case it isn’t just about the iterator, but also what you want to get back. If you’re just after some matching strings, re.findall() would be easier to use as you get the strings themselves back. If you want more info about the match, you may want to use re.finditer() even if it isn’t a lot of data, because if gives you the match objects.

Great Course! Enjoyed learning lots of new stuff using regex. Thank you.

Great Course! Thank You!

What a complex but great course. Are there any books you recommend to learn more about this?

Hi Andre,

I asked my colleagues and there were two suggestions:

“Regular Expressions” by Jeffrey Friedl – it is a bit older, but the tech hasn’t changed

and as strange as this might sound on a Python site:

“Teach Yourself Perl in 21 Days” by Laura Lemay

Perl uses regexes heavily, so you may find useful stuff in that book.

I haven’t read either of these, but trust the folks making the recommendations.

Late to the party, but in case anyone else reading the comments is interested, one more recommendation from another RP guy:

learnbyexample.github.io/books/#python-re-gex

Loved it. Just the right level for a 80+ programmer.