Rust Platform

🎯 sed 's/old/new/' is the find-and-replace you reach for daily (also works in Vim: :s/old/new/).

Two things define it: it works one line at a time, and by default it swaps only the first match on each line. Add the g (global) flag - s/old/new/g - to swap every match.

In Python you'd lean on str.replace, whose third argument is the replacement count:

def substitute(text: str, frm: str, to: str, global_: bool) -> str:
    count = -1 if global_ else 1            # -1 = all, 1 = just the first
    return "\n".join(line.replace(frm, to, count) for line in text.splitlines())

Rust gives you the same two behaviours as two different methods, and you choose between them.

str has a real replace — unlike tr

In the tr bite you will write text a char at a time by hand. Here the unit is a substring, and &str hands you the method outright:

let shout = "na na na".replace("na", "YO");   // "YO YO YO" — every match

replace swaps all occurrences. Its sibling replacen takes a count and stops early:

let once = "na na na".replacen("na", "YO", 1); // "YO na na" — just the first

So the global flag is just a choice between these two, no manual counting needed.

sed is line-oriented, so split, transform, rejoin

The catch: "first match" means first match per line, not first in the whole text.

So you can't run one replace over everything; you process each line on its own.

• .lines() gives you that iterator (you met it in cat -n and sort),
• .map() transforms each line,
• and .join() stitches the results back into one String:

let parts = ["one", "two", "three"];
let line = parts.join(", ");                   // "one, two, three"

.lines() drops the newlines on the way in, so rejoining with "\n" rebuilds the text.

Mapping with a branch inside

Each line gets the same decision — all matches or just the first — driven by the global flag. An if/else is an expression in Rust, so it can be the whole body of the closure, returning one String either way:

let label = |n: i32| if n % 2 == 0 { "even" } else { "odd" };   // both arms: &str