use tr instead of gsub when possible

[acrogenesis]

tr is over 5x faster than gsub, in this 2 cases they are interchangeable

[matthewd]

Thanks, but I'm closing this as cosmetic: both of those occur adjacent to much more expensive operations, that will make any speed difference here insignificant.

[fxn]

Hey, reopening this one. tr is the concise idiomatic method call you do for that kind of things. Applying because in my mind this is better code, regardless of performance.