input          12345
output         gsub(/regex/,"x",input)

regex          output
(^)            x12345
($)            12345x
(^)|($)        x12345x
($)|(^)        x12345x
(2)            1x345
(^)|2          x1x345
2|(^)          x1x345
($)|2          1x345x
2|($)          1x345x
(2)|(^)        x1x345
(^)|(2)        x1x345
(2)|($)        1x345x
($)|(2)        1x345x
.((2)|(^))     x345
.((^)|(2))     x345
.((2)|($))     x34x
.(($)|(2))     x34x
x*((2)|(^))    x1x345
x*((^)|(2))    x1x345
x*((2)|($))    1x345x
x*(($)|(2))    1x345x