Regex: Matching by exclusion, without look-ahead – is it possible?

by

UPDATE: It fails “with two ff before oo” as @Ciantic pointed out in the comments.

^(f(o[^o]|[^o])|[^f])*$

NOTE: It is much much easier just to negate a match on the client side instead of using the above regex.

The regex assumes that each line ends with a newline char if it is not then see C++’s and grep’s regexs.

Sample programs in Perl, Python, C++, and grep all give the same output.

  • perl

    #!/usr/bin/perl -wn
print if /^(f(o[^o]|[^o])|[^f])*$/;

  • python

    #!/usr/bin/env python
import fileinput, re, sys
from itertools import ifilter

re_not_foo = re.compile(r"^(f(o[^o]|[^o])|[^f])*$")
for line in ifilter(re_not_foo.match, fileinput.input()):
    sys.stdout.write(line)

  • c++

    #include <iostream>
#include <string>
#include <boost/regex.hpp>

int main()
{
  boost::regex re("^(f(o([^o]|$)|([^o]|$))|[^f])*$");
  //NOTE: "|$"s are there due to `getline()` strips newline char

  std::string line;
  while (std::getline(std::cin, line)) 
    if (boost::regex_match(line, re))
      std::cout << line << std::endl;
}

  • grep

    $ grep "^\(f\(o\([^o]\|$\)\|\([^o]\|$\)\)\|[^f]\)*$" in.txt

Sample file:

foo
'foo'
abdfoode
abdfode
abdfde
abcde
f

fo
foo
fooo
ofooa
ofo
ofoo

Output:

abdfode
abdfde
abcde
f

fo
ofo

More Related Contents:

  1. Regular expression to match a line that doesn’t contain a word
  2. Regular expression for a string containing one word but not another
  3. How to negate the whole regex?
  4. Regular expression that doesn’t contain certain string [duplicate]
  5. How to exclude a specific string constant? [duplicate]
  6. Regex: match pattern as long as it’s not in the beginning
  7. Regex – Does not contain certain Characters
  8. How can I match a pattern as long as it’s not at the beginning with regex?
  9. Negate characters in Regular Expression [closed]
  10. Regex to get the words after matching string [duplicate]
  11. Unix grep regex containing ‘x’ but not containing ‘y’
  12. Regular Expression to exclude set of Keywords
  13. How do I turn any regex into an complement of itself without complex hand editing?
  14. How to extract all Yahoo emails from a .CSV file?
  15. Regular expression to remove fancy apostrophe [closed]
  16. What is a word boundary in regex?
  17. Escaping a forward slash in a regular expression
  18. Regex: Determine if two regular expressions could match for the same input?
  19. In regular expressions, what is a backtracking / back referencing?
  20. Comma Separated Numbers Regex
  21. Use a function in Powershell replace
  22. RegEx pattern any two letters followed by six numbers
  23. A regex for version number parsing
  24. Fixing Catastrophic Backtracking in Regular Expression
  25. apache HTTP:X-Forwarded-Proto in .htaccess is causing redirect loop in dev environment
  26. Using RegEX To Prefix And Append In Notepad++
  27. How to match the first word after an expression with regex?
  28. When is it best to use Regular Expressions over basic string splitting / substring’ing?
  29. How to make “grep” read patterns from a file?
  30. How to measure similarity between strings?

Leave a Comment