Question

I would like to match strings that end with bar, for example: foobar or bar. Such regexp could be: /^.*bar$/.

I would also like to exclude strings with the letter u prefixed to bar, for instance, these strings should not match the regular expression: ubar or fooubar. I tried /^.*[^u]?bar$/, but it doesn’t work. How could we fix this?

Answer 1

Simply wrap the whole prefix in parentheses

^(.*[^u])?bar$

By doing this you only allow further preceding characters, if there was at least one non-u character before bar.

Alternatively, if your regex engine supports negative lookbehinds, you could do this:

^.*(?<!u)bar$

When this regex reaches the position before bar it looks at the character left of it and tries to match a u. If that is not possible, the match continues. If the u was found the lookbehind will make the pattern fail. This works both if there is a non-u character and if it’s the beginning of the string.

As sawa pointed out in a comment, you don’t even need the ^.* if you just want to check whether a string ends in bar:

(?<!u)bar$

Of course, if you want to include the whole string in the match for some reason (replacement or matching lines using multiline mode) then the ^.* is necessary. Note that in the first regex you cannot leave it out. However you could change it to

([^u]|^)bar$

Which would also avoid matching the whole string.