Regex to remove leading zeros in R, unless the final (or only) character is zero

You could use an alternation to either match all the zeroes in the string in a capturing group or match all the zeroes from the start of the string.

In the replacement use group 1.

^0*(0)$|^0+

Regex demo | R demo

For example

sub("^0*(0)$|^0+", "\\1", c("005", "0AB", "000", "0"))

Output

[1] "5"  "AB" "0"  "0"

Or even better as commented by Wiktor Stribiżew, you could use capture a single 0 in a group and repeat the group itself to capture the last instance of a zero.

^(0)+$|^0+

Regex demo

You may remove all zeros from the start of a string but not the last one:

sub("^0+(?!$)", "", x, perl=TRUE)

See the regex demo.

Details

• ^ - start of a string
• 0+ - one or more zeros
• (?!$) - a negative lookahead that fails the match if there is an end of string position immediately to the right of the current location

See the R demo:

x <- c("005", "0AB", "000", "0")
sub("^0+(?!$)", "", x, perl=TRUE)
## => [1] "5"  "AB" "0"  "0"

We can add one more condition with a regex lookaround to check for any non-zero values after one or more zeros (0+)

sub("(?<![0-9])0+(?=[^0])", "", sub("^0+$", "0", v1), perl = TRUE)
#[1] "5"  "AB" "0"  "0" 

data

v1 <- c("005", "0AB", "000", "0")