parse a string using regex

Question

I have a string

txt = 'text1 & ("text2" | "text3" | "text4") & "text5" ! (text6 | text7 | text8)'

Lets say I want to parse it so I end up with elements that are between parenthesis. My pattern looks like

pattern = '\(([^\)(]+)\)'

using python I end up with two groups

>>> print re.findall(pattren, text)
['"text2" | "text3" | "text4"', 'text6 | text7 | text8']

Lets say we want to find some thing like

>>> print re.findall(magic_pattren, text )
['& ("text2" | "text3" | "text4")', '! (text6 | text7 | text8)']

Any guesses on what that magic_pattren would be. I can work my way to the desired output using string operations.

 >>> print [txt[str.find(txt, a)-3: 1+len(a)+str.find(txt, a)] for a in re.findall(pattren, txt)]
 ['& ("text2" | "text3" | "text4")', '! (text6 | text7 | text8)']

But this feels a bit clunky and fails if the parenthesis group is in the beginning. I can add a check on that, but like I said feels a bit clunky. Any takers?

Please check this demo with the r'\B\W\s*\([^()]+\)' regex. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Mar 8, 2016 at 8:06
Thanks for that, but this will miss if the group is in start. ideone.com/eWvumU — fahad daniyal
– fahad daniyal, Commented Mar 8, 2016 at 8:12

Wiktor Stribiżew · Accepted Answer · 2016-03-08 08:19:47Z

2

You can use the (?:\B\W\s*)? optional group at the beginning of the pattern:

import re
p = re.compile(r'(?:\B\W\s*)?\([^()]+\)')
test_str = "(text9 & text10) & text1 & (\"text2\" | \"text3\" | \"text4\") & \"text5\" ! (text6 | text7 | text8)"
print(p.findall(test_str))

Result of the sample demo: ['(text9 & text10)', '& ("text2" | "text3" | "text4")', '! (text6 | text7 | text8)']

The (?:\B\W\s*)? is a non-capturing group (so that the value is not output in the result) that can be repeated one or zero times (due to the last ?), and it matches a non-word character (\W) only if it is preceded with a non-word character or start of string (\B) and followed with 0+ whitespace.

Here is the regex demo

answered Mar 8, 2016 at 8:19

Wiktor Stribiżew

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