使用Python匹配对象和特定正则表达式

Question

Given a text, I need to check for each char if it has 3 capital letters on both sides and if there are, add it to a string of such characters that is retured.

I wrote the following: m = re.match("[AZ]{3}.[AZ]{3}", text) (let's say text="AAAbAAAcAAA")

I expected to get two groups in the match object: "AAAbAAA" and "AAAcAAA"

Now, When i invoke m.group(0) I get "AAAbAAA" which is right. Yet, when invoking m.group(1) , I find that there is no such group, meaning "AAAcAAA" wasn't a match. Why?

Also, when invoking m.groups() , I get an empty tuple although I should get a tuple of the matches, meaning that in my case I should have gotten a tuple with "AAAbAAA". Why doesn't that work?

Answer 1

You don't have any groups in your pattern. To capture something in a group, you have to surround it with parentheses:

([A-Z]{3}).[A-Z]{3}

The exception is m.group(0) , which will always contain the entire match.

Looking over your question, it sounds like you aren't actually looking for capture groups, but rather overlapping matches. In regex, a group means a smaller part of the match that is set aside for later use. For example, if you're trying to match phone numbers with something like

([0-9]{3})-([0-9]{3}-[0-9]{4})

then the area code would be in group(1) , the local part in group(2) , and the entire thing would be in group(0) .

What you want is to find overlapping matches. Here's a Stack Overflow answer that explains how to do overlapping matches in Python regex , and here's my favorite reference for capture groups and regex in general.

Answer 2

One, you are using match when it looks like you want findall . It won't grab the enclosing capital triplets, but re.findall('[AZ]{3}([az])(?=[AZ]{3})', search_string) will get you all single lower case characters surrounded on both sides by 3 caps.