Regular Expression¶

Ha Khanh Nguyen (hknguyen)

1. Basic Components
2. Putting It Together

1. Basic Components¶

Regular expression, or regex, is built using the following components:
We can use regex101.com to try out our regex!

Anchors: ^ and $¶

Regex	Functionality	Matched Strings
`^x`	matches any string that starts with x	'xoom', 'xx', 'x097', etc.
`x$`	matches any string that ends with x	'ox', 'xx', '098x', etc.
`^x$`	exact string match (has to be exactly 'x')	'x'
`x`	matches any string that has `x` in it	'xoom', 'ox', 'xx', 'x097', '098x', '1x1', 'x', etc.

Quantifiers: * + ? and {}¶

Regex	Functionality	Matched Strings
`abc*`	matches any string that has `ab` followed by 0 or more `c`	'abc', 'ab', 'abccc', 'aaaab', etc.
`abc+`	matches any string that has `ab` followed by 1 or more `c`	'abc', 'abccc', 'aabc', etc.
`abc?`	matches any string that has `ab` followed by 0 or 1 `c`	'ab', 'abc', 'abbbbbbb', 'ababc', etc.
`abc{2}`	matches any string that has `ab` followed by exactly 2 `c`	'abcc', 'aabcc', etc.
`abc{2,}`	matches any string that has `ab` followed by 2 or more `c`	'abcc', 'aabccc', etc.
`abc{2, 5}`	matches any string that has `ab` followed by 2 up to 5 `c`	'abcc', 'aabccccc', etc.
`a(bc)*`	matches any string that has `a` followed by 0 or more copies of the sequence `bc`	'a', 'abc', 'aaabcbc', etc.
`a(bc){2, 5}`	matches any string that has `a` followed by 2 up to 5 copies of the sequence `bc`	'abcbc', 'aaabcbcbc'

OR operation: | or []¶

Regex	Functionality	Matched Strings
`a(b\|c)`	matches any string that has `a` followed by `b` or `c` (and captures b or c)	'ab', 'ac', 'aaaabc', etc.
`a[bc]`	same as above but without capturing b or c }	'ab', 'ac', 'aaaabc', etc.

Character classes: \d \w \s and .¶

Regex	Functionality	Matched Strings
`\d`	matches a single character that is a digit	'4', '2', '9', '0', etc.
`\w`	matches a word character (alphanumeric character plus underscore)	'_', 'h', '4', etc.
`\s`	matches a whitespace character (includes tabs and line breaks)	' ', '\t', '\n', etc.
`.`	matches any character	' ', '?', 'a', etc.

\d, \w, and \s also present their negations with \D, \W, and \S respectively.
That is, \D matches a single non-digit character.

Escape character: \¶

In order to be taken literally, you must escape the characters ^.[S()|*+?{\ with a backslash \ as they have special meaning.
For example, \$\d matches a string that has a $ before one digit.

Flags¶

We use flag to signal different type of matching:
- g (global) does not return after the first match, restarting the subsequent searches from the end of the previous match.
- m (multi-line) when enabled ^ and $ will match the start and end of a line (which ends with \n).
- i (insensitive) makes the whole expression not case-sensitive.

Grouping and capturing: ()¶

Regex	Functionality
`a(bc)`	parentheses create a capturing group with value `bc`
`a(?:bc)*`	using `?:` we disable the capturing group
`a(?<foo>bc)`	using `?<foo>` we put a name to the group

This is extremely useful when we need to extract data from the string.

Bracket expressions: []¶

Regex	Functionality	Matched Strings
`[abc]`	matches any string that has either `a` or `b` or `c` (same as `a\|b\|c`)	'a', 'debt', 'wwwwc', etc.
`[a-c]`	same as above	'a', 'debt', 'wwwwc', etc.
`[a-zA-Z0-9]`	matches any string that represents a single digit or alphabet letter (not case-sensitive)	'a', '9', 'H', etc.
`[^a-zA-Z]`	matches any string that has not a letter from a to z (not case sensitive)	'7', '898', etc.

2. Putting It Together¶

Write a regex that checks the validity of a Social Security Number (SSN).
Write a regex that checks for a valid email address.

This lecture notes reference materials from Johnny Fox's blog post Regex tutorial - A quick cheatsheet by examples on medium.com.