[Tut] Your Python Regex Pattern Doesn’t Match? Try This!

[Tut] Your Python Regex Pattern Doesn’t Match? Try This! - Printable Version

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[Tut] Your Python Regex Pattern Doesn’t Match? Try This! - xSicKxBot - 05-27-2022

Your Python Regex Pattern Doesn’t Match? Try This!

5/5 – (1 vote)

Problem Formulation

Say, you want to find a regex pattern in a given string. You know the pattern exists in the string. You use the

Code:
re.match(pattern, string)

function to find the match object where the pattern matches in the string.

Problem: The Python regular expression pattern is not found in the string. The pattern doesn’t match anything, and, thus, the match object is

Code:
None

. How to fix this?

Here’s an example in which you’re searching for the pattern

Code:
'h[a-z]+'

which should match the substring

Code:
'hello'

.

But it doesn’t match!

Code:
import re my_string = 'hello world'
pattern = re.compile('h[a-z]+') match = re.match(pattern, my_string) if match: print('found!')
else: print('not found!')

Output:

Code:
not found!

Where is the bug? And how to fix it, so that the pattern matches the substring

Code:
'hello'

Learn More: Improve your regex superpower by studying character classes used in the example pattern

Code:
'h[a-z]+'

by visiting this tutorial on the Finxter blog.

Solution: Use re.search() instead of re.match()

A common reason why your Python regular expression pattern is not matching in a given string is that you mistakenly used

Code:
re.match(pattern, string)

instead of

Code:
re.search(pattern, string)

Code:
re.findall(pattern, string)

. The former attempts to match the

Code:
pattern

at the beginning of the

Code:
string

, whereas the latter two functions attempt to match anywhere in the string.

Here’s a quick recap of the three regex functions:

Code:
re.match(pattern, string)
returns a match object if the
Code:
pattern
matches at the beginning of the
Code:
string
. The match object contains useful information such as the matching groups and the matching positions.
Code:
re.search(pattern, string)
matches the first occurrence of the
Code:
pattern
in the
Code:
string
and returns a matchobject.
Code:
re.findall(pattern, string)
scans
Code:
string
from left to right, searching for all non-overlapping matches of the
Code:
pattern
. It returns a list of strings in the matching order when scanning the string from left to right.

Thus, the following code uses re.search() to fix our problem:

Code:
import re my_string = 'hello world'
pattern = re.compile('h[a-z]+') match = re.search(pattern, my_string) if match: print('found!')
else: print('not found!')

Output:

Code:
found!

Finally, the pattern

Code:
'h[a-z]+'

does match the string

Code:
'hello world'

.

Note that you can also use the re.findall() function if you’re interested in just the string matches of your pattern (without match object). We’ll explain all of this — re.match(), re.search(), re.findall(), and match objects — in a moment but first, let’s have a look at the same example with re.findall():

Code:
import re my_string = 'hello world'
pattern = re.compile('h[a-z]+') match = re.findall(pattern, my_string) print(match)
# ['hello'] if match: print('found!')
else: print('not found!')

Output:

Code:
['hello']
found!

Understanding re.match()

The

Code:
re.match(pattern, string)

method returns a match object if the

Code:
pattern

matches at the beginning of the

Code:
string

. The match object contains useful information such as the matching groups and the matching positions. An optional argument

Code:
flags

allows you to customize the regex engine, for example to ignore capitalization.

Specification:

Code:
re.match(pattern, string, flags=0)

The

Code:
re.match()

method has up to three arguments.

Code:
pattern
: the regular expression pattern that you want to match.
Code:
string
: the string which you want to search for the pattern.
Code:
flags
(optional argument): a more advanced modifier that allows you to customize the behavior of the function. Want to know how to use those flags? Check out this detailed article on the Finxter blog.

We’ll explore them in more detail later.

Return Value:

The

Code:
re.match()

method returns a match object. You may ask (and rightly so):

Learn More: Understanding

Code:
re.match()

on the Finxter blog.

What’s a Match Object?

If a regular expression matches a part of your string, there’s a lot of useful information that comes with it: what’s the exact position of the match? Which regex groups were matched—and where?

The match object is a simple wrapper for this information. Some regex methods of the re package in Python—such as

Code:
search()

—automatically create a match object upon the first pattern match.

At this point, you don’t need to explore the match object in detail. Just know that we can access the start and end positions of the match in the string by calling the methods

Code:
m.start()

and

Code:
m.end()

on the match object

Code:
m

Code:
>>> m = re.search('h...o', 'hello world')
>>> m.start()
0
>>> m.end()
5
>>> 'hello world'[m.start():m.end()] 'hello'

In the first line, you create a match object m by using the

Code:
re.search()

method. The pattern

Code:
'h...o'

matches in the string

Code:
'hello world'

at start position 0.

You use the start and end position to access the substring that matches the pattern (using the popular Python technique of slicing).

Now that you understood the purpose of the match object, let’s have a look at the alternative to the

Code:
re.match()

function next!

Understanding re.search()

The

Code:
re.search(pattern, string)

method matches the first occurrence of the

Code:
pattern

in the

Code:
string

and returns a matchobject.

Specification:

Code:
re.search(pattern, string, flags=0)

The

Code:
re.search()

method has up to three arguments.

Code:
pattern
: the regular expression pattern that you want to match.
Code:
string
: the string which you want to search for the pattern.
Code:
flags
(optional argument): a more advanced modifier that allows you to customize the behavior of the function. Want to know how to use those flags? Check out this detailed article on the Finxter blog.

We’ll explore them in more detail later.

Return Value:

The

Code:
re.search()

method returns a match object. You may ask (and rightly so):

Learn More: Understanding

Code:
re.search()

on the Finxter blog.

Understanding re.findall()

The

Code:
re.findall(pattern, string)

method scans

Code:
string

from left to right, searching for all non-overlapping matches of the

Code:
pattern

. It returns a list of strings in the matching order when scanning the string from left to right.

[Image: refindall-1024x576.jpg]

Specification:

Code:
re.findall(pattern, string, flags=0)

The

Code:
re.findall()

method has up to three arguments.

Code:
pattern
: the regular expression pattern that you want to match.
Code:
string
: the string which you want to search for the pattern.
Code:
flags
(optional argument): a more advanced modifier that allows you to customize the behavior of the function. Want to know how to use those flags? Check out this detailed article on the Finxter blog.

We will have a look at each of them in more detail.

Return Value:

The

Code:
re.findall()

method returns a list of strings. Each string element is a matching substring of the string argument.

Learn More: Understanding

Code:
re.findall()

on the Finxter blog.

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