Python regex: matching a parenthesis within parenthesis

Question

I’ve been trying to match the following string:

string = "TEMPLATES = ( ('index.html', 'home'), ('base.html', 'base'))"

But unfortunately my knowledge of regular expressions is very limited, as you can see there are two parentheses that need to be matched, along with the content inside the second one
I tried using re.match("(w*)", string) but it didn’t work, any help would be greatly appreciated.

Asked By: Paulo

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Source

Answer 1

Your sample is looking for open paren followed by zero or more letter w followed by close paren. You probably want to use w instead of w, but that won’t work in your case anyway, because you have non-word characters next to the open paren.

I think you should consider splitting the string at the commas instead. What is your final objective?

Answered By: Joseph Bui

Answer 2

First of all, using ( isn’t enough to match a parenthesis. Python normally reacts to some escape sequences in its strings, which is why it interprets ( as simple (. You would either have to write \( or use a raw string, e.g. r'(' or r"(".

Second, when you use re.match, you are anchoring the regex search to the start of the string. If you want to look for the pattern anywhere in the string, use re.search.

Like Joseph said in his answer, it’s not exactly clear what you want to find. For example:

string = "TEMPLATES = ( ('index.html', 'home'), ('base.html', 'base'))"
print re.findall(r'([^()]*)', string)

will print

["('index.html', 'home')", "('base.html', 'base')"]

EDIT:

I stand corrected, @phooji is right: escaping is irrelevant in this specific case. But re.match vs. re.search or re.findall is still important.

Answered By: Vojislav Stojkovic

Answer 3

Try this:

import re
w = "TEMPLATES = ( ('index.html', 'home'), ('base.html', 'base'))"

# find outer parens
outer = re.compile("((.+))")
m = outer.search(w)
inner_str = m.group(1)

# find inner pairs
innerre = re.compile("('([^']+)', '([^']+)')")

results = innerre.findall(inner_str)
for x,y in results:
    print("%s <-> %s" % (x,y))

Output:

index.html <-> home
base.html <-> base

Explanation:

outer matches the first-starting group of parentheses using ( and ); by default search finds the longest match, giving us the outermost ( ) pair. The match m contains exactly what’s between those outer parentheses; its content corresponds to the .+ bit of outer.

innerre matches exactly one of your ('a', 'b') pairs, again using ( and ) to match the content parens in your input string, and using two groups inside the ' ' to match the strings inside of those single quotes.

Then, we use findall (rather than search or match) to get all matches for innerre (rather than just one). At this point results is a list of pairs, as demonstrated by the print loop.

Update: To match the whole thing, you could try something like this:

rx = re.compile("^TEMPLATES = (.+)")
rx.match(w)

Answered By: phooji

Answer 4

If your strings look like valid Python code anyways you can do this:

import ast
var, s = [part.strip() for part in 
     "TEMPLATES = ( ('index.html', 'home'), ('base.html', 'base'))".split('=')]
result= ast.literal_eval(s)

Answered By: Jochen Ritzel

Answer 5

In case you want to validate that parentheses are balanced up two levels deep, you can use this regular expression:

import re;

string = """( ('index.html', 'home'), ('base.html', 'base'))
('index.html', 'home')
('base.html', 'base')
"""

pattern = re.compile(r"(?P<expression>(([^()]*(?P<parenthesis>()(?(parenthesis)[^()]*)))*?[^()]*))")

match = pattern.findall(string)

print(match[0][0])
print(match[1][0])
print(match[2][0])

This regular expression uses conditional statement (?(parenthesis)[^()]*)).

Demo: https://repl.it/@Konard/ParenthesesExample

Answered By: Konard

Python regex: matching a parenthesis within parenthesis

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