Regex: except when

Ruby
1

Hello,
I haven’t grokked how to include an exception to a condition in a
regular expression. For example, I can’t express this correctly:

aString matches if it has a sequence of 3 or more consonants, except
when the consonants [ng] is in the sequence (in that order, n followed
by g).

Thus: plants will match, but bangking will not match.

Thanks for the help,
basi

2

basi wrote:

Hello,
I haven’t grokked how to include an exception to a condition in a
regular expression. For example, I can’t express this correctly:

aString matches if it has a sequence of 3 or more consonants, except
when the consonants [ng] is in the sequence (in that order, n followed
by g).

Thus: plants will match, but bangking will not match.

You might find negative look-ahead helpful. For example:

consonant = ‘[bcdfghjklmnpqrstvwxyz]’
r = /#{consonant}{3,}(?!.*ng)/

print "plants: ", r =~ “plants”, “\n”
print "bangking: ", r =~ “bangking”, “\n”

3

Thanks much! I haven’t tried it yet, but it looks so elegant it must
work.
basi

4

basi wrote:

Thanks much! I haven’t tried it yet, but it looks so elegant it must
work.

One thing I screwed up: If you want to avoid strings that contain ng
anywhere, do the negative look-ahead before you match the consonants:

consonant = ‘[bcdfghjklmnpqrstvwxyz]’
r = /(?!.*ng)#{consonant}{3,}/

5

Yes, this one catches initial and final occurences as well. Much
appreciated.
basi