CODEWITHSUNDEEP
pythonregexpython-2.7
linuxfan
linuxfan

Reputation: 1160

Capturing portions of text with non-greedy regex

Using re.findall, I want to extract the values assigned to each PCR.

>>> z
'PCR-09: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\nPCR-10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\nPCR-11: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\nPCR-12: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\nPCR-13: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\nPCR-14: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\nPCR-15: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\nPCR-16: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r\n

>>> print z
PCR-09: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
PCR-10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
PCR-11: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
PCR-12: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
PCR-13: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
PCR-14: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
PCR-15: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 
PCR-16: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

Initially, I tried this but can someone point out what is wrong with the regex used?

>>> re.search('PCR-09:(.*?)', z).groups()
('',)

Shouldn't the non-greedy expr (.*?) match all characters until it finds a newline?

With a slightly modified regex, I get the desired result:

>>> re.search('PCR-09:(.*?)\s\r\n', z).groups()
(' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00',)

On the same lines, this doesn't work:

>>> re.findall(r'(PCR-\d+):(.*?)', z)
[('PCR-09', ''), ('PCR-10', ''), ('PCR-11', ''), ('PCR-12', ''), ('PCR-13', ''), ('PCR-14', ''), ('PCR-15', ''), ('PCR-16', ''),

But this does:

>>> re.findall(r'(PCR-\d+):(.*?)\s\r\n', z,re.DOTALL)
[('PCR-09', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'), ('PCR-10', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'), ('PCR-11', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'), ('PCR-12', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'), ('PCR-13', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'), ('PCR-14', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'), ('PCR-15', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'), ('PCR-16', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00'),

Hoping someone can explain what's wrong with my approach.

Thanks

Upvotes: 1

Views: 51

Answers (3)

Hackaholic
Hackaholic

Reputation: 19743

try using: split

[ x.split(':') for x in z.split('\r\n')]

output:

[['PCR-09', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['PCR-10', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['PCR-11', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['PCR-12', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['PCR-13', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['PCR-14', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['PCR-15', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['PCR-16', ' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 '], ['']]

using regex

re.findall('(PCR-\d+)(.*)',z)

Upvotes: 0

Christophe Biocca
Christophe Biocca

Reputation: 3468

The reason r'PCR-09:(.*?)' doesn't match what you expect is that non-greedy regexes stop as soon as they're valid.

So (.*?) can match '', so the regex stops immediately.

In contrast, r'(PCR-\d+):(.*?)\s\r\n' is non-greedy, but because it needs to find `\s\r\n', it will force the expansion to work.

I would recommend using a greedy regex that includes only the characters you expect to find: r'(PCR-\d+):([0-9 ]*)'.

Upvotes: 3

user2555451
user2555451

Reputation:

The pattern PCR-09:(.*?) tells Python to match zero or more characters non-greedily after PCR-09:. So, it does precisely this and matches zero characters.

You need to have your Regex be greedy in order to match everything up to the newline:

>>> re.search('PCR-09:(.*)', z).groups()
(' 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \r',)
>>>

Note that your PCR-09:(.*?)\s\r\n pattern worked because it told Python to get zero or more characters after PCR-09: and up to \s\r\n. In other words, get everything between them.

Upvotes: 2

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