Python backreference replacing doesn't work as expected

Question

There are two named groups in my pattern: myFlag and id, I want to add one more myFlag immediately before group id.

Here is my current code:

# i'm using Python 3.4.2
import re
import os
contents = b'''
xdlg::xdlg(x_app* pApp, CWnd* pParent)
    : customized_dlg((UINT)0, pParent, pApp)
    , m_pReaderApp(pApp)
    , m_info(pApp)
{

}
'''

pattern = rb'(?P<myFlag>[a-zA-Z0-9_]+)::(?P=myFlag).+:.+(?P<id>\(UINT\)0 *,)'
res = re.search(pattern, contents, re.DOTALL)
if None != res:
    print(res.groups()) # the output is (b'xdlg', b'(UINT)0,')

# 'replPattern' becomes b'(?P<myFlag>[a-zA-Z0-9_]+)::(?P=myFlag).+:.+((?P=myFlag)\\(UINT\\)0 *,)'
replPattern = pattern.replace(b'?P<id>', b'(?P=myFlag)', re.DOTALL)
print(replPattern)
contents = re.sub(pattern, replPattern, contents)
print(contents)

The expected results should be:

xdlg::xdlg(x_app* pApp, CWnd* pParent)
    : customized_dlg(xdlg(UINT)0, pParent, pApp)
    , m_pReaderApp(pApp)
    , m_info(pApp)
{

}

but now the result this the same with the original:

 xdlg::xdlg(x_app* pApp, CWnd* pParent)
    : customized_dlg((UINT)0, pParent, pApp)
    , m_pReaderApp(pApp)
    , m_info(pApp)
{

}

Why are you replacing ?P<id> (no parens) with (?P=myFlag) (with parens)? — Martijn Pieters
– Martijn Pieters, Commented Jul 12, 2015 at 1:58
Besides, there is no third xdlg string in the input. What exactly are you trying to achieve here? — Martijn Pieters
– Martijn Pieters, Commented Jul 12, 2015 at 1:59
because I want to replace the string with 'myFlag' group, it's a must to encloe them with parens(python syntax) — O'Skywalker
– O'Skywalker, Commented Jul 12, 2015 at 2:00
What is the expected output here? What problem are you trying to solve? — Martijn Pieters
– Martijn Pieters, Commented Jul 12, 2015 at 2:00
You replaced the name of a named group with a back reference, the regex syntax makes no sense here. — Martijn Pieters
– Martijn Pieters, Commented Jul 12, 2015 at 2:01

l'L'l · Accepted Answer · 2015-07-12 03:07:33Z

2

The issue appears to be the pattern syntax — particularly the end:

0 *,)

That makes no sense really... fixing it seems to solve most of the issues, although I would recommend ditching DOTALL and going with MULTILINE instead:

p = re.compile(ur'([a-zA-Z0-9_]+)::\1(.*\n\W+:.*)(\(UINT\)0,.*)', re.MULTILINE)
sub = u"\\1::\\1\\2\\1\\3"
result = re.sub(p, sub, s)

print(result)

Result:

xdlg::xdlg(x_app* pApp, CWnd* pParent)
    : customized_dlg(xdlg(UINT)0, pParent, pApp)
    , m_pReaderApp(pApp)
    , m_info(pApp)
{

}

https://regex101.com/r/hG3lV7/1

edited Jul 12, 2015 at 3:07

answered Jul 12, 2015 at 2:37

l'L'l

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6 Comments

O'Skywalker Over a year ago

i had tried your pattern, it did work. however, i'm confused, is it necessary to introduce 5 groups? is it possbile to solve this issue with 2 groups?

l'L'l Over a year ago

No five groups aren't necessary (see edit), although I couldn't entirely make sense of the ones you had either. In my opinion named groups in Python are somewhat worthless unless you have many groups that you need to keep track of across multiple replacements, etc. Otherwise it's just more code that essentially makes things more confusing.

l'L'l Over a year ago

It's not necessary, so remove it. I think the pattern can be further improved too — the answer in this instance was to mainly to illustrate what the issue was for it not working.

O'Skywalker Over a year ago

how can I achieve the same effect with re.DOTALL?

l'L'l Over a year ago

re.DOTALL should work with the pattern above (still using three capture groups), although could be simplified to be something like ([a-zA-Z0-9_]+)::\1(.*)(\(UINT\)0,.*).

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