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