473,320 Members | 1,961 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,320 software developers and data experts.

regular expression for nested parentheses

I have been trying to write a regular expression that identifies a
block of text enclosed by (potentially nested) parentheses. I've found
solutions using other regular expression engines (for example, my text
editor, BBEdit, which uses the PCRE library), but have not been able
to replicate it using python's re module.

Here's a version that works using the PCRE syntax, along with the
python error message. I'm hoping for this to identify the string '(foo
(bar) (baz))'

% python -V
Python 2.5.1
% python
pyimport re
pytext = 'buh (foo (bar) (baz)) blee'
pyno_ws = lambda s: ''.join(s.split())
pyrexp = r"""(?P<parens>
.... \(
.... (?>
.... (?[^()]+ ) |
.... (?P>parens)
.... )*
.... \)
.... )"""
pyprint re.findall(no_ws(rexp), text)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Library/Frameworks/Python.framework/Versions/2.5/lib/
python2.5/re.py", line 167, in findall
return _compile(pattern, flags).findall(string)
File "/Library/Frameworks/Python.framework/Versions/2.5/lib/
python2.5/re.py", line 233, in _compile
raise error, v # invalid expression
sre_constants.error: unexpected end of pattern

From what I understand of the PCRE syntax, the (?>) construct is a non-
capturing subpattern, and (?P>parens) is a recursive call to the
enclosing (named) pattern. So my best guess at a python equivalent is
this:

pyrexp2 = r"""(?P<parens>
.... \(
.... (?=
.... (?= [^()]+ ) |
.... (?P=parens)
.... )*
.... \)
.... )"""
pyprint re.findall(no_ws(rexp2), text)
[]

....which results in no match. I've played around quite a bit with
variations on this theme, but haven't been able to come up with one
that works.

Can anyone help me understand how to construct a regular expression
that does the job in python?

Thanks -

Dec 9 '07 #1
5 8228
On Dec 10, 8:13 am, Noah Hoffman <noah.hoff...@gmail.comwrote:
I have been trying to write a regular expression that identifies a
block of text enclosed by (potentially nested) parentheses. I've found
solutions using other regular expression engines (for example, my text
editor, BBEdit, which uses the PCRE library), but have not been able
to replicate it using python's re module.
A pattern that can validly be described as a "regular expression"
cannot count and thus can't match balanced parentheses. Some "RE"
engines provide a method of tagging a sub-pattern so that a match must
include balanced () (or [] or {}); Python's doesn't.

Looks like you need a parser; try pyparsing.

[snip]
pyrexp = r"""(?P<parens>
... \(
... (?>
... (?[^()]+ ) |
... (?P>parens)
... )*
... \)
... )"""
pyprint re.findall(no_ws(rexp), text)
Ummm ... even if Python's re engine did do what you want, wouldn't you
need flags=re.VERBOSE in there?

Dec 9 '07 #2
On Dec 9, 1:41 pm, John Machin <sjmac...@lexicon.netwrote:
A pattern that can validly be described as a "regular expression"
cannot count and thus can't match balanced parentheses. Some "RE"
engines provide a method of tagging a sub-pattern so that a match must
include balanced () (or [] or {}); Python's doesn't.
Okay, thanks for the clarification. So recursion is not possible using
python regular expressions?
Ummm ... even if Python's re engine did do what you want, wouldn't you
need flags=re.VERBOSE in there?
Ah, thanks for letting me know about that flag; but removing
whitespace as I did with the no_ws lambda expression should also work,
no?
Dec 9 '07 #3
On Dec 10, 8:53 am, Noah Hoffman <noah.hoff...@gmail.comwrote:
On Dec 9, 1:41 pm, John Machin <sjmac...@lexicon.netwrote:
A pattern that can validly be described as a "regular expression"
cannot count and thus can't match balanced parentheses. Some "RE"
engines provide a method of tagging a sub-pattern so that a match must
include balanced () (or [] or {}); Python's doesn't.

Okay, thanks for the clarification. So recursion is not possible using
python regular expressions?
Ummm ... even if Python's re engine did do what you want, wouldn't you
need flags=re.VERBOSE in there?

Ah, thanks for letting me know about that flag; but removing
whitespace as I did with the no_ws lambda expression should also work,
no?
Under a very limited definition of "work". That technique would not
produce correct answers on patterns that contain any *significant*
whitespace e.g. you want to match "foo" and "bar" separated by one or
more spaces (but not tabs, newlines etc) ....
pattern = r"""
foo
[ ]+
bar
"""
Dec 9 '07 #4
On Dec 9, 10:12 pm, John Machin <sjmac...@lexicon.netwrote:
On Dec 10, 8:53 am, Noah Hoffman <noah.hoff...@gmail.comwrote:
On Dec 9, 1:41 pm, John Machin <sjmac...@lexicon.netwrote:
A pattern that can validly be described as a "regular expression"
cannot count and thus can't match balanced parentheses. Some "RE"
engines provide a method of tagging a sub-pattern so that a match must
include balanced () (or [] or {}); Python's doesn't.
Okay, thanks for the clarification. So recursion is not possible using
python regular expressions?
Ummm ... even if Python's re engine did do what you want, wouldn't you
need flags=re.VERBOSE in there?
Ah, thanks for letting me know about that flag; but removing
whitespace as I did with the no_ws lambda expression should also work,
no?

Under a very limited definition of "work". That technique would not
produce correct answers on patterns that contain any *significant*
whitespace e.g. you want to match "foo" and "bar" separated by one or
more spaces (but not tabs, newlines etc) ....
pattern = r"""
foo
[ ]+
bar
"""
You can also escape a literal space:

pattern = r"""
foo
\ +
bar
"""
Dec 10 '07 #5
On Dec 10, 12:22 pm, MRAB <goo...@mrabarnett.plus.comwrote:
On Dec 9, 10:12 pm, John Machin <sjmac...@lexicon.netwrote:


On Dec 10, 8:53 am, Noah Hoffman <noah.hoff...@gmail.comwrote:
On Dec 9, 1:41 pm, John Machin <sjmac...@lexicon.netwrote:
A pattern that can validly be described as a "regular expression"
cannot count and thus can't match balanced parentheses. Some "RE"
engines provide a method of tagging a sub-pattern so that a match must
include balanced () (or [] or {}); Python's doesn't.
Okay, thanks for the clarification. So recursion is not possible using
python regular expressions?
Ummm ... even if Python's re engine did do what you want, wouldn't you
need flags=re.VERBOSE in there?
Ah, thanks for letting me know about that flag; but removing
whitespace as I did with the no_ws lambda expression should also work,
no?
Under a very limited definition of "work". That technique would not
produce correct answers on patterns that contain any *significant*
whitespace e.g. you want to match "foo" and "bar" separated by one or
more spaces (but not tabs, newlines etc) ....
pattern = r"""
foo
[ ]+
bar
"""

You can also escape a literal space:

pattern = r"""
foo
\ +
bar
"""
I know that. *Any* method of putting in a literal significant space is
clobbered by the OP's "trick" of removing *all* whitespace instead of
using the VERBOSE flag, which also permits comments:
pattern = r"""
\ + # ugly
[ ]+ # not quite so ugly
"""
Dec 10 '07 #6

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

1
by: Kenneth McDonald | last post by:
I'm working on the 0.8 release of my 'rex' module, and would appreciate feedback, suggestions, and criticism as I work towards finalizing the API and feature sets. rex is a module intended to make...
5
by: joemono | last post by:
Hello everyone! First, I appologize if this posting isn't proper "netiquette" for this group. I've been working with perl for almost 2 years now. However, my regular expression knowledge is...
11
by: Martin Robins | last post by:
I am trying to parse a string that is similar in form to an OLEDB connection string using regular expressions; in principle it is working, but certain character combinations in the string being...
9
by: MJ | last post by:
HI I want to know what is mean by regular expression in C Mayur
10
by: Lee Kuhn | last post by:
I am trying the create a regular expression that will essentially match characters in the middle of a fixed-length string. The string may be any characters, but will always be the same length. In...
5
by: Tony Marston | last post by:
I am seeking help with a regular expression that will split a string into several parts with ',' (comma) as the separator, but NOT where the separator is enclosed in parentheses. For example, take...
9
by: a | last post by:
I need to write a regular expression to match a quoted string in which the double quote character itself is represented by 2 double quotes. For example: "beginning ""nested quoted string"" end"...
16
by: Mark Rae | last post by:
Hi, Supposing I had a string made up of a person's name followed by their profession in parentheses e.g. string strText = "Tiger Woods (golfer)"; and I wanted to extract the portion of the...
5
by: shawnmkramer | last post by:
Anyone every heard of the Regex.IsMatch and Regex.Match methods just hanging and eventually getting a message "Requested Service not found"? I have the following pattern: ^(?<OrgCity>(+)+),...
0
by: DolphinDB | last post by:
The formulas of 101 quantitative trading alphas used by WorldQuant were presented in the paper 101 Formulaic Alphas. However, some formulas are complex, leading to challenges in calculation. Take...
0
by: ryjfgjl | last post by:
ExcelToDatabase: batch import excel into database automatically...
0
by: Vimpel783 | last post by:
Hello! Guys, I found this code on the Internet, but I need to modify it a little. It works well, the problem is this: Data is sent from only one cell, in this case B5, but it is necessary that data...
0
by: jfyes | last post by:
As a hardware engineer, after seeing that CEIWEI recently released a new tool for Modbus RTU Over TCP/UDP filtering and monitoring, I actively went to its official website to take a look. It turned...
0
by: ArrayDB | last post by:
The error message I've encountered is; ERROR:root:Error generating model response: exception: access violation writing 0x0000000000005140, which seems to be indicative of an access violation...
1
by: PapaRatzi | last post by:
Hello, I am teaching myself MS Access forms design and Visual Basic. I've created a table to capture a list of Top 30 singles and forms to capture new entries. The final step is a form (unbound)...
0
by: CloudSolutions | last post by:
Introduction: For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
0
by: Defcon1945 | last post by:
I'm trying to learn Python using Pycharm but import shutil doesn't work
0
by: Shællîpôpï 09 | last post by:
If u are using a keypad phone, how do u turn on JavaScript, to access features like WhatsApp, Facebook, Instagram....

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.