# Working With Regex Expressions In Python

> This post will look at how the regex module in python can be used to work with regular expressions.

- Canonical: https://dennisokeeffe.com/blog/2021-08-02-working-with-regex-expressions-in-python
- Published: 2021-08-02
- Updated: 2021-08-02
- Tags: beginner, python, 100daysofpython

> This is Day 14 of the #100DaysOfPython challenge

This post will use the [regular expressions module](https://docs.python.org/3/library/re.html) from the standard library to ... .

## Prerequisites

1. Familiarity with [Pipenv](https://pipenv.pypa.io/en/latest/). See [here](https://blog.dennisokeeffe.com/blog/2020-07-04-abcs-of-pipenv) for my post on Pipenv.
2. Familiarity with [JupyterLab](https://jupyterlab.readthedocs.io/en/stable/gettingstarted/install.html). See [here](https://blog.dennisokeeffe.com/blog/2021-07-19-hello-jupyter-notebook) for my post on JupyterLab.
3. Familiarity with [Regular Expressions](https://www.regular-expressions.info/)

## Getting started

Let's create the `hello-python-regex` directory and install Pillow.

```s
# Make the `hello-python-regex` directory
$ mkdir hello-python-regex
$ cd hello-python-regex
# Create a folder to place your icons
$ mkdir icons

# Init the virtual environment
$ pipenv --three
$ pipenv install --dev jupyterlab
```

At this stage, we can start up the notebook server.

```s
# Startup the notebook server
$ pipenv run jupyter-lab
# ... Server is now running on http://localhost:8888/lab
```

The server will now be up and running.

## Creating the notebook

Once on http://localhost:8888/lab, select to create a new Python 3 notebook from the launcher.

Ensure that this notebook is saved in `hello-python-regex/docs/regex.ipynb`.

We will explore the following in each cell of the notebook:

1. Importing the Regex module.
2. A basic usage of the Regex module.
3. String replacement with the Regex module.

## Importing the module

This imports the [regex module](https://docs.python.org/3/library/re.html) from the standard library.

```py
import re

m = re.search("Hello, (.+)", "Hello, world!")
m.group(1)
# 'world!'
```

## Basic usage of the module

There are a number of useful module methods that we can use that we will demonstrate:

1. Searching strings.
2. Matching strings.
3. Usage without compile.
4. Splitting a string into a list.
5. Replacing matches.

## Searching strings

> Scan through string looking for the first location where this regular expression produces a match, and return a corresponding match object. Return None if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string.

```py
import re

pattern = re.compile("ello, (.+)")
m = pattern.search("Hello, world!")
m.group(1)
print(m) # <re.Match object; span=(0, 13), match='Hello, world!'>
print(m.group(1)) # world!

n = pattern.search("Hello, world!", 0)
print(n) # <re.Match object; span=(0, 13), match='Hello, world!'>
print(n.group(1)) # world!
```

## Matching strings

> If zero or more characters at the beginning of string match this regular expression, return a corresponding match object. Return None if the string does not match the pattern; note that this is different from a zero-length match.

```py
pattern = re.compile("ello, (.+)")
m = pattern.match("Hello, world!")
# No match as "e" is the 2nd character the "Hello, world!".
print(m) # None

pattern = re.compile("Hello, (.+)")
# Does match
n = pattern.match("Hello, world!")
print(n) # <re.Match object; span=(0, 13), match='Hello, world!'>
```

## Usage without compile

When you use `re.match` and `re.search` as a static method, you can pass the Regex as the first argument:

```py
m = re.match("Hello, (.+)", "Hello, world!")
print(m) # <re.Match object; span=(0, 13), match='Hello, world!'>

n = re.match("Hello, (.+)", "Hello, world!")
print(n) # <re.Match object; span=(0, 13), match='Hello, world!'>
```

## Splitting a string into a list

```py
m = re.split(",", "Hello, world!")

print(m) # ['Hello', ' world!']

n = re.split("\s", "Hello beautiful world!")

print(n) # ['Hello', 'beautiful', 'world!']
```

## Replacing matches

We can make use of the `search` and `sub` methods to replace matches.

```py
# Simple example
target = "Photo credit by [@thomas](https://site.com/@thomas)"
m = re.search(r"Photo credit by \[@(.+)\]\(https://site.com/@(.+)\)", target)
res = re.sub(m.group(1), "dennis", target)
print(res) # Photo credit by [@dennis](https://site.com/@dennis)

# By iterating for multiple matches
target = "Photo credit by [@thomas](https://site.com/@user)"
m = re.search(r"Photo credit by \[@(.+)\]\(https://site.com/@(.+)\)", target)
res = target

for idx, val in enumerate(m.groups()):
		res = re.sub(val, "dennis", res)

print(res) # Photo credit by [@dennis](https://site.com/@dennis)
```

For a more specific replacement (particularly in a large set of text), we can be more explicit with the string to replace:

```py
target = """
Other words thomas and user we don't want to replace.
Photo credit by [@thomas](https://site.com/@user)
"""
new_name = "dennis"

pattern = re.compile(r"Photo credit by \[@(.+)\]\(https://site.com/@(.+)\)")
res = pattern.sub(f"Photo credit by [@{new_name}](https://site.com/@{new_name})", target)
# Other words thomas and user we don't want to replace.
# Photo credit by [@dennis](https://site.com/@dennis)
```

## Summary

Today's post demonstrated how to use the `re` module from the standard library to search, match, split and replace text in Python strings.

This can be unbelievably useful when working with text files.

## Resources and further reading

1. [The ABCs of Pipenv](https://blog.dennisokeeffe.com/blog/2020-07-04-abcs-of-pipenv)
2. [Hello, JupyterLab](https://blog.dennisokeeffe.com/blog/2021-07-19-hello-jupyter-notebook)
3. [Pipenv](https://pipenv.pypa.io/en/latest/)
4. [Regular Expressions in Python](https://docs.python.org/3/library/re.html)
5. [Info on Regular Expressions](https://www.regular-expressions.info/)
6. [Regex 101 - website for testing Regex expressions](https://regex101.com/)

_Photo credit: [`pawel_czerwinski`](https://unsplash.com/@pawel_czerwinski)_