generate-negative-regex.md

Details

This StackOverflow answer shows how to generate mathematiclly proven regular expression to match a line that doesn't contain a word.

If you want to do the same, you need to run vcsn locally since vcsn sandbox doesn't work anymore.

How-to

1. Start vcsn using Docker

docker run --rm -p 8888:8888 lrde/vcsn

2. Open http://localhost:8888
3. Create new notebook
4. Insert new cell
5. Add this code to the cell

import vcsn

# your string
input = 'nomatch'

# use proper charset
c = vcsn.context('lal_char(a-z), b')
e = c.expression('(%s){c}' % input)
a = e.automaton()

print(
    '^(' +
    str(
        a.expression()
    ).replace('+','|')  # + is usually denoted |
    .replace('\e','') # \e denotes the empty word
    .replace('[^]','.') # [^] is usually written .
    + ')$'
)

6. Run notebook
7. Simplify generated regex: http://ivanzuzak.info/noam/webapps/regex_simplifier/