regex a before b with and without whitespaces

I am trying to find all the string which include ‘true’ when there is no ‘act’ before it.

An example of possible vector:

vector = c("true","trueact","acttrue","act true","act really true")

What I have so far is this:

grepl(pattern="(?<!act)true", vector, perl=T, ignore.case = T)
[1]  TRUE  TRUE FALSE  TRUE  TRUE

what I’m hopping for is

[1]  TRUE  TRUE FALSE  FALSE  FALSE  

>Solution :

Here is one way to do so:

grepl(pattern="^(.(?<!act))*?true", vector, perl=T, ignore.case = T)
[1]  TRUE  TRUE FALSE FALSE FALSE

• ^: start of the string
• .: matches any character
• (?<=): negative lookbehind
• act: matches act
• *?: matches .(?<!act) between 0 and unlimited times
• true: matches true

see here for the regex demo