R regexp for sharing text parts

Question

I want to swap text parts from left to right and vice versa. This works fine from left to right:

sub("(^x+)(.+)", "\\2\\1","xxxxx6.0")
[1] "6.0xxxxx"

while in the other direction there is:

sub("(.+)(x+$)", "\\2\\1","6.0xxxxx")
[1] "x6.0xxxx"

What am I missing?

Answer 1

The problem is your second regular expression. The second regular expression has .+, which is the greedy quantifier corresponding to each character. The first group will try to match as much as possible.

(6.0xxxx)(x)

In parentheses are two groups that will match your regular expression.

There are two ways to solve this problem. The first is to use a lazy quantifier instead of a greedy quantifier:

/(.+?)(x+$)/

+ , , .

(6.0)(xxxxx)

.

, , , x.

/(^[^x]+)(+x$)/

, ( , x). x 1, .

Answer 2

lookbehind (?<!...) , , (.+) . lookbehind , (x+$) , x:

sub("(.+)(?<!x)(x+$)", "\\2\\1", "6.0xxxxx", perl=TRUE)
#[1] "xxxxx6.0"