Expression Will match . . .
foo The string "foo"
^foo "foo" at the start of a line
foo$ "foo" at the end of a line
^foo$ "foo" when it is alone on a line
[Ff]oo "Foo" or "foo"
[abc] a, b, or c
[^abc] d, e, f, g, V, %, ~, 5, etc.everything that is not a, b, or c (^ is "not" inside character classes)
[A-Z] Any uppercase letter
[a-z] Any lowercase letter
[A-Za-z] Any letter
[A-Za-z0-9] Any letter or number
[A-Z]+ One or more uppercase letters
[A-Z]* Zero or more uppercase letters
[A-Z]? Zero or one uppercase letters
[A-Z]{3} Three uppercase letters
[A-Z]{3,} A minimum of three uppercase letters
[A-Z]{1,3} One, two, or three uppercase letters
[^0-9] Any non-numeric character
[^0-9A-Za-z] Any symbol (not a number or a letter)
(cat|sat) Matches either "cat" or "sat"
([A-Z]{3}|[0-9]{4}) Matches three letters or four numbers
Fo* F, Fo, Foo, Fooo, Foooo, etc.
Fo+ Fo, Foo, Fooo, Foooo, etc.
Fo? F, Fo
. Any character except \n (new line)
\b A word boundary; e.g. te\b matches the "te" in "late" but not the "te" in "tell."
\B A non-word boundary; "te\B" matches the "te" in "tell" but not the "te" in "late."
\n Newline character
\s Any whitespace (new line, space, tab, etc.)
\S Any non-whitespace character