How can I exclude a specific string from from python regex

Question

I'm attempting to parse /etc/mtab but exclude /boot. I thought perhaps non-capturing groups would be the way to go, but it doesn't work as I expected. This is the regex I constructed:

proc = subprocess.Popen(["ssh", server, "cat", mtab],stdout = subprocess.PIPE)
for line in proc.stdout:
  fsMatch = re.search(r'([\w/:]+) (/([\w/:-]+)|(?:boot)) (nfs|ext3)', line)
  if fsMatch:
    print fsMatch.group(1,2,4)

Output:

('/dev/sda1', '/boot', 'ext3')
('/dev/mapper/foo1', '/export/foo1', 'ext3')
('/dev/mapper/foo2', '/export/foo2', 'ext3')
('/dev/mapper/foo3', '/export/foo3', 'ext3')
('/dev/mapper/foo4', '/export/foo4', 'ext3')
('/dev/mapper/foo5', '/export/foo5', 'ext3')
('servernfs:/install', '/mnt', 'nfs')

I'm pretty confident the | is wrong (and obviously more is wrong) but have hit a roadblock.

I'm looking for all matches for /[\w/:-]+ but exclude matches to /boot

Suggestions?

Regular expressions are not good at excluding things. Do it in a separate check. — Barmar
– Barmar, Commented Apr 4, 2013 at 6:01

hobs · Accepted Answer · 2013-04-04 06:25:15Z

6

You need to use a negative lookbehind or negative lookahead, described here with a hint below:

r'^(?!/boot).*$'

If you need to capture that 'servernfs:' one and not 'servernfs:/boot', you'll need to sprinkle in a little '|' and '([a-z]+:)' somewhere at the top (after the '^')

answered Apr 4, 2013 at 6:25

hobs

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Comments

Burhan Khalid · Accepted Answer · 2013-04-04 06:11:53Z

1

Just exclude the line:

for line in proc.stdout:
  if 'boot' not in line:
      # the rest

However, since mtab is delimited on space, you can just use split:

>>> with open('foo.txt') as f:
...     lines = [line.split(' ') for line in f if 'boot' not in line]
...

edited Apr 4, 2013 at 6:11

answered Apr 4, 2013 at 6:03

Burhan Khalid

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3 Comments

Jon Clements Over a year ago

It'd make more sense for that check to go on the if fsMatch line wouldn't it? eg:if fsMatch and not fsMatch.group(2).startswith('/boot') ?

hobs Over a year ago

Yea, the startswith is key. But then you've got the fs:/boot matching problem.

hobs Over a year ago

paths like /home/user/bin/reboot will be rejected by the 'boot' not in line filter, I think. And '/boot' doesn't fix it either without the startswith bit. I like Jon Clements approach. Let's the regex do the heavy lifting of a match and startswith does the simple, precise exclude.

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