grep

Line anchors

We use line anchors to specify the exact position in the file where the regular expression should be matched. We use ^ to match regex at the beginning of the line, and $ to match regex at the end of the line.
If we had a file containing these words:

cat dog concatenate dogma category educated boondoggle vindication chilidog

^cat would match only cat and category, and dog$ would match dog and chilidog

. is an unrestricted wildcard character, eg. c.t would match cat, c5t or cQt.
set of character [character] would, in the case of c[aou]t, match patterns that start with c. and are followed by an a, or an o, or an u, followed by t. gr8.

* means zero or infinitely many repetitions of the character behind it. We usually use it together with . and we write it as .* so that it means none or infinitely many (*) of any character (.).

Important :D Difference between (), [] and {}

[] represent a character class, while () represent character group.
[abc] would match a, or b, or c. (abc) would match exactly these three character abc

For example:
a -- Can be captured by [a-z0-9].
a-z0-9 -- Can be captured by (a-z0-9) and then can be referenced in a replacement and/or later in the expression

Curly brackets { } are general repetition quantifiers. They specify a minimum and maximum number of permitted matches. {match the string if at least n times, match the string if not more than n times}

For example: a{2,4} matches aa, aaa, or aaaa, but not a or aaaaa

{n} - exactly n times

{n,} - no maximum limit

Question mark

Outside a character class, a question mark (?) matches zero or one character in the string. It is the same as using {0,1}.

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We use regular expression with grep

$ grep 'c[aou]t' list.txt
$ grep 'cat$' list.txt
$ ps aux | grep spring

grep has several useful options for adjusting how it matches regular expressions:
-i match regex as case-insensitive
-v only show lines that DO NOT match the regex
-r apply recursively to a group of files or dirs -A <NUMBER> display the <NUMBER> of lines after the regex match
-B <NUMBER> display the <NUMBER> of lines before the regex match
-e match several regexes can be supplied and they will be matched with logical OR

$ grep '^[#;]' dogs-n-cats will match lines that begin with # or ;.
$ grep '[^#;]' dogs-n-cats, in this case, ^ means negation, so it will match everything that's not # or ;.