Regex 101

A primer on writing regexes

Introduction

Matching the beginning of a line using the caret ^

The caret or ^ matches the beginning of the line. Anything coming after the caret will only match if it's present at the beginning of any line in a multiline string.

`^abc` matches "abc" only if it appears at the beginning of a string or line.
`^Hello` will match the string "Hello" only if it is at the very beginning of a line.

The dot operator .

A dot . matches any single character other than an newline \n. A dot can be used a wildcard for any single character except for a newline character. This makes it very versatile for matching any character in a specific position within a string.

`a.b` will match any string that has an "a", followed by any single character (except a newline), followed by a "b". For example, it will match "acb", "a1b", and "a-b".

`...` will match any three characters in a row (except for newlines). For example, it will match "abc", "123", and "a b".

The end operator $

The end operator $ is used to denote the end of a string or the end of line. This is useful for ensuring that a match occurs only at the end of a string or line. The $ operator in regex is not a character but a positional assertion that denotes the end of a line or string.

`abc$` will match the string "abc" only if it appears at the very end of a string.
It will match "xabc" but not "abcy".

regex: abc$
string: "xabc\nyabc\nzabc"
matches: "xabc", "yabc", "zabc" (all three "abc" at the end of each line)

The asterisk or star operator *

The asterisk * is a quantifier that matches the preceding element zero or more times. This means that the element it follows can appear any number of times, including not at all.

regex: ba*n
strings: "bn", "ban", "baan", "baaan"
matches: "bn", "ban", "baan", "baaan"

regex: a.*b
strings: "ab", "acb", "axyzb", "a123b"
matches: "ab", "acb", "axyzb", "a123b" (Since . matches any character other than $ and * represents 0 or more times)

The digit matcher \d and the non digit matcher \D

The \d used to match any digits (0-9). The \D operator is used to match any non-digit characters.

The following regex matches a sequence of 2 digits, 1 non-digit, 2 digit, 1 non-digit and 4 digits.

"\d\d\D\d\d\D\d\d\d\d"

The \D will match any whitespace operator too since they are non digits.

The whitespace matcher \s and the non whitespace matcher \S

The \s matches nay of the white space characters (\r, \n, \t, \f). The \S matches any non whitespace character.

The string "12 11 15" matches the regex "\S\S\s\S\S\s\S\S"

The word \w and the non word \W

The \w matches a word character, it could be a letter, a digit or an underscore. The \W matches a non-word character, anything that other than what \w matches.

The following pattern matches
# 3 word characters, 1 non-words, 10 word characters, 1 non-word character and finally 3 word characters.

"\w\w\w\W\w\w\w\w\w\w\w\w\w\w\W\w\w\w"

The {} quantifier

The {} quantifier is used to specify the exact number of occurrences, or a range of occurrences, of the preceding element. The syntax within the braces allows for various forms of repetition.

The string "aaa" matches regex "a{3}", whereas "aa" wil not match it. 

The string "12 11 15" matches the regex "\S{2}\s\S{2}\s\S{2}" and the regex "\d{2}\D\d{2}\D\d{2}".

When paired with , within the {}, ranges or "atleast" logic can be used.

At Least n Occurrences:
{n,}: Matches at least n occurrences of the preceding element.
Example: a{2,} matches "aa", "aaa", "aaaa", etc.

Range of Occurrences:
{n,m}: Matches between n and m occurrences (inclusive) of the preceding element.
Example: a{2,4} matches "aa", "aaa", and "aaaa".