# Regular Expressions

> Source: https://learn-python.com/regular-expressions/
> Part of Learn Python, free to read.

Regular Expressions (sometimes shortened to regexp, regex, or re) are a
tool for matching patterns in text. In Python, we have the re module.
The applications for regular expressions are wide-spread, but they are
fairly complex, so when contemplating using a regex for a certain task,
think about alternatives, and come to regexes as a last resort.

An example regex is `r"^(From|To|Cc).*?python-list@python.org"` Now for an
explanation:
the caret `^` matches text at the beginning of a line. The following
group, the part with `(From|To|Cc)` means that the line has to start with
one of the words that are separated by the pipe `|`. That is called
the OR operator, and the regex will match if the line starts with any
of the words in the group. The `.*?` means to un-greedily match any
number of characters, except the newline `\n` character. The un-greedy
part means to match as few repetitions as possible. The `.` character
means any non-newline character, the `*` means to repeat 0 or more
times, and the `?` character makes it un-greedy.

So, the following lines would be matched by that regex:
`From: python-list@python.org`
`To: !asp]<,. python-list@python.org`

A complete reference for the re syntax is available at the [python
docs](http://docs.python.org/library/re.html#regular-expression-syntax
“RE syntax).

As an example of a “proper” email-matching regex (like the one in the
exercise), see [this](http://www.ex-parrot.com/pdw/Mail-RFC822-Address.html)