18

Here's something I'm trying to do with regular expressions, and I can't figure out how. I have a big file, and strings abc, 123 and xyz that appear multiple times throughout the file.

I want a regular expression to match a substring of the big file that begins with abc, contains 123 somewhere in the middle, ends with xyz, and there are no other instances of abc or xyz in the substring besides the start and the end.

Is this possible with regular expressions?

Jorge Campos
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Ram Rachum
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    [Since regular expressions are not fully standardized, all questions with this tag should also include a tag specifying the applicable programming language or tool.](http://stackoverflow.com/tags/regex/info) That said, is there any particular reason you want to use regular expressions here? It's possible, but in most environments, it's more complicated than not using regexes. –  May 15 '16 at 15:56
  • Should line breaks be considered or not? The big file will be read line by line or as one big string? – Jorge Campos May 15 '16 at 15:59
  • Regex flavor is python 2.7, newlines should be included. – Ram Rachum May 15 '16 at 16:16

4 Answers4

37

When your left- and right-hand delimiters are single characters, it can be easily solved with negated character classes. So, if your match is between a and c and should not contain b (literally), you may use (demo)

a[^abc]*c

This is the same technique you use when you want to make sure there is a b in between the closest a and c (demo):

a[^abc]*b[^ac]*c

When your left- and right-hand delimiters are multi-character strings, you need a tempered greedy token:

abc(?:(?!abc|xyz|123).)*123(?:(?!abc|xyz).)*xyz

See the regex demo

To make sure it matches across lines, use re.DOTALL flag when compiling the regex.

Note that to achieve a better performance with such a heavy pattern, you should consider unrolling it. It can be done with negated character classes and negative lookaheads.

Pattern details:

  • abc - match abc
  • (?:(?!abc|xyz|123).)* - match any character that is not the starting point for a abc, xyz or 123 character sequences
  • 123 - a literal string 123
  • (?:(?!abc|xyz).)* - any character that is not the starting point for a abc or xyz character sequences
  • xyz - a trailing substring xyz

See the diagram below (if re.S is used, . will mean AnyChar):

enter image description here

See the Python demo:

import re
p = re.compile(r'abc(?:(?!abc|xyz|123).)*123(?:(?!abc|xyz).)*xyz', re.DOTALL)
s = "abc 123 xyz\nabc abc 123 xyz\nabc text 123 xyz\nabc text xyz xyz"
print(p.findall(s))
// => ['abc 123 xyz', 'abc 123 xyz', 'abc text 123 xyz']
Wiktor Stribiżew
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3

Using PCRE a solution would be:

This using m flag. If you want to check only from start and end of a line add ^ and $ at beginning and end respectively 

abc(?!.*(abc|xyz).*123).*123(?!.*(abc|xyz).*xyz).*xyz

Regular expression visualization

Debuggex Demo

Jorge Campos
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2

The comment by hvd is quite appropriate, and this just provides an example. In SQL, for instance, I think it would be clearer to do:

where val like 'abc%123%xyz' and
      val not like 'abc%abc%' and
      val not like '%xyz%xyz'

I imagine something quite similar is simple to do in other environments.

Jonathan Leffler
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Gordon Linoff
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1

You could use lookaround.

/^abc(?!.*abc).*123.*(?<!xyz.*)xyz$/g

(I've not tested it.)

Kenny Lau
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