- ^ matches the start of a string.
- $ matches the end of a string.
- . matches all characters except a newline.
- | means or, so a|b means it will match a or b. If it matches a first, then b will not be tried.
- \ is an escape character, which is used to match special characters that usually have other meanings. An example would be \$ that matches a literal $ rather than the end of a string.

Basic Characters

- The + character matches the expression at least 1 time.
- The *  character matches the expression 0 or more times.
- ? matches the expression 0 or 1 time.
- {x} matches the expression exactly x times.
- {x, y} matches the expression between x and y times.
- {x,} matches the expression at least x times.
- {,y} matches the expression at most y times.

Quantifying Characters

- \w matches any alphanumeric character, so a-z, A-Z, 0-9 and _.
- \W matches non-alphanumeric characters.
- \d matches digits (0-9).
- \D matches any non-digits.
- \s matches whitespace characters, including newlines, tabs, or spaces.
- \S matches non-whitespace characters.
- \n matches newline characters.
- \t matches tab characters.

Character Classes

- [] is used to group characters/expressions together.
```python
#matches a,b, or c and NOT abc
[abc]
#matches any capital letters from A to Z
[A-Z]
#matches characters from a to z or digits 0 to 9
[a-z0-9]
#Using ^ in the set excludes characters in the set
#matches any character not x, y, or 4
[^xy4]
```

Character Sets

Here is a non-exhaustive list of recognized special characters when using regular expressions in python. The documentation contains a more complete list of these characters:

- .
- *
- +
- ?
- {x}
- {x, y}
- \
- []
- |
- \d
- \s
- \w

University of Chicago

The re library allows the user to perform matches with regular expressions.

https://docs.python.org/3/library/re.html
https://docs.python.org/3/howto/regex.html

re documentation

Special Characters

Here is a list of commonly used methods:

- match()
- search()
- findall()
- finditer()

re Methods

You can compile a regular expression for use in the following way:

```python
import re

r = re.compile("\d\d\d")


re: Compiling a regular expression

https://www.geeksforgeeks.org/python-regex-cheat-sheet/

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