r - Replace every single character at the start of string that matches a regex pattern

Question

I want to replace every single character in a string that matches a certain pattern. Take the following string

mystring <- c("000450")

I want to match all single zeros up to the first element that is non-zero. I tried something like

gsub("^0[^1-9]*", "x", mystring)
[1] "x450"

This expression replaces all the leading zeros with a single x. But instead, I want to replace all three leading zeros with xxx. The preferred result would be

[1] "xxx450"

Can anyone help me out?

Answer 1

You may use

mystring <- c("000450")
gsub("\G0", "x", mystring, perl=TRUE)
## => [1] "xxx450"

See the regex demo and an R demo

The \G0 regex matches 0 at the start of the string, and any 0 that only appears after a successful match.

Details

• G - an anchor that matches ("asserts") the position at the start of the string or right after a successful match
• 0 - a 0 char.