Regex in Python - Using groups

Question

I am new to regex, why does this not output 'present'?

tale = "It was the best of times, ... far like the present ... of comparison only. "
a = re.compile('p(resent)')
print a.findall(tale)

>>>>['resent']

coz the group your regex is searching is 'resent' here. can you provide a better example of expected input/output? — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Dec 25, 2012 at 2:55
I expected it to output the string 'present' in a list of one element ('present' only occurs once in the string, tale). I believed that I was searching for instances of 'resent' preceded by a 'p'. — Chet
– Chet, Commented Dec 25, 2012 at 2:58
expected output for this : tale="resent present ppresent presentt"? — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Dec 25, 2012 at 3:04
The output I expected from the above line was: ['resent','present'] — Chet
– Chet, Commented Dec 25, 2012 at 3:14

Ashwini Chaudhary · Accepted Answer · 2012-12-25 03:08:17Z

3

try something like this if you're trying to match the exact word present here:

In [297]: tale="resent present ppresent presentt"

In [298]: re.findall(r"\bpresent\b",tale)
Out[298]: ['present']

edited Dec 25, 2012 at 3:08

answered Dec 25, 2012 at 3:01

Ashwini Chaudhary

252k60 gold badges478 silver badges519 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Barmar · Accepted Answer · 2012-12-25 03:06:34Z

1

From the Python documentation

If one or more groups are present in the pattern, return a list of groups

If you want it to just use the group for grouping, but not for capturing, use a non-capture group:

a = re.compile('p(?:resent)')

For this regular expression, there's no point in it, but for more complex regular expressions it can be appropriate, e.g.:

a = re.compile('p(?:resent|eople)')

will match either 'present' or 'people'.

answered Dec 25, 2012 at 3:06

Barmar

789k57 gold badges554 silver badges669 bronze badges

6 Comments

Chet Over a year ago

Ok, cool. This explains how to do what I was trying to do. Can you explain to me what my previous code was looking for (why was the p disregarded)? Thank you very much.

Barmar Over a year ago

It was being used in matching, but not being captured in the return info. An example would be if you wanted to find a percentage, but only cared about the number part -- you would use the regex (\d+)%.

Chet Over a year ago

So once parentheses are included, only things inside the parenthesis are captured?

Anorov Over a year ago

@Chet, when using re.findall, yes. If no parentheses are found then the entire string will be the result element. When using search or match, you can specify the entire string by using result.group(0) or access the groups with result.group(1) etc.

Blue Magister Over a year ago

@Barmar I'm not too familiar with Python regex, but could OP have used two groups in his/her regex? Say if a = re.compile('(p(resent))')?

|

Collectives™ on Stack Overflow

Regex in Python - Using groups

2 Answers 2

Comments

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related