Use sed to match a regular expression and replace the full line with just the match.

MatchAndReplaceLine.sh

#!/bin/bash

# Use \( and \) to create a match group in the find part, use \1 in the replace part to reference the match group.

echo "./ThingToMatch/other/stuff/" | sed -n 's/\.\/\([a-zA-Z0-9]*\)\/.*/\1/p'

# This will output: ThingToMatch
# Works with multiline input, applying the replacement to each line.