BASICS
Literal text
A regex is a search pattern. The simplest pattern is the text itself.
The pattern cat finds the three characters c, a, t in a row. Most characters mean exactly themselves - you type the text you are hunting for and the engine finds every place it appears.
A pattern matches anywhere inside the text, so cat also matches the middle of "concatenate". Limiting matches to whole words comes later. (These drills always match globally - every occurrence is highlighted.)
Match every occurrence of the word cat, including when it sits inside a longer word.
/ /
my cat sleeps all day
must match: "cat"
concatenate the strings
must match: "cat"
cat catalog cathedral
must match: "cat" "cat" "cat"
a dog barks
must match nothing