A regex to match a substring that isn't followed by a certain other substring
I need a regex that will match blahfooblah but not blahfoobarblah
I want it to match only foo and everything around foo, as long as it isn't followed by bar.
I tried using this: foo.*(?<!bar) which is fairly close, but it matches blahfoobarblah. The negative look behind needs to match anything and not just bar.
The specific language I'm using is Clojure which uses Java regexes under the hood.
EDIT: More specifically, I also need it to pass blahfooblahfoobarblah but not blahfoobarblahblah.
Answer
Try:
/(?!.*bar)(?=.*foo)^(\w+)$/
Tests:
blahfooblah # pass
blahfooblahbarfail # fail
somethingfoo # pass
shouldbarfooshouldfail # fail
barfoofail # fail
Regular expression explanation
NODE EXPLANATION
--------------------------------------------------------------------------------
(?! look ahead to see if there is not:
--------------------------------------------------------------------------------
.* any character except \n (0 or more times
(matching the most amount possible))
--------------------------------------------------------------------------------
bar 'bar'
--------------------------------------------------------------------------------
) end of look-ahead
--------------------------------------------------------------------------------
(?= look ahead to see if there is:
--------------------------------------------------------------------------------
.* any character except \n (0 or more times
(matching the most amount possible))
--------------------------------------------------------------------------------
foo 'foo'
--------------------------------------------------------------------------------
) end of look-ahead
--------------------------------------------------------------------------------
^ the beginning of the string
--------------------------------------------------------------------------------
( group and capture to \1:
--------------------------------------------------------------------------------
\w+ word characters (a-z, A-Z, 0-9, _) (1 or
more times (matching the most amount
possible))
--------------------------------------------------------------------------------
) end of \1
--------------------------------------------------------------------------------
$ before an optional \n, and the end of the
string
Other regex
If you only want to exclude bar when it is directly after foo, you can use
/(?!.*foobar)(?=.*foo)^(\w+)$/
Edit
You made an update to your question to make it specific.
/(?=.*foo(?!bar))^(\w+)$/
New tests
fooshouldbarpass # pass
butnotfoobarfail # fail
fooshouldpassevenwithfoobar # pass
nofuuhere # fail
New explanation
(?=.*foo(?!bar)) ensures a foo is found but is not followed directly bar