Python Regex Word Boundary
Summary: in this tutorial, you’ll learn how to construct regular expressions that match word boundary positions in a string.
Introduction to the Python regex word boundary
A string has the following positions that qualify as word boundaries:
- Before the first character in the string if the first character is a word character (
\w
). - Between two characters in the string if the first character is a word character (
\w
) and the other is not (\W
– inverse character set of the word character\w
). - After the last character in a string if the last character is the word character (
\w
)
The following picture shows the word boundary positions in the string "PYTHON 3!"
:
In this example, the "PYTHON 3!"
string has four word boundary positions:
- Before the letter P (criteria #1)
- After the letter N (criteria #2)
- Before the digit 3 (criteria #2)
- After the digit 3 (criteria #2)
Regular expressions use the \b
to represent a word boundary. For example, you can use the \b
to match the whole word
using the following pattern:
r'\bword\b'
Code language: JavaScript (javascript)
The following example matches the word Python
in a string:
import res = 'CPython is the implementation of Python in C'
matches = re.finditer('Python', s)
for match in matches:
print(match.group())
Code language: JavaScript (javascript)
It returns two matches, one in the word CPython
and another in the word Python
.
Python
Python
However, if you use the word boundary \b
, the program returns one match:
import res = 'CPython is the implementation of Python in C'
matches = re.finditer(r'\bPython\b', s)
for match in matches:
print(match.group())
Code language: JavaScript (javascript)
Output:
<re.Match object; span=(33, 39), match='Python'>
Code language: HTML, XML (xml)
In this example, the '\bPython\b'
pattern match the whole word Python
in the string 'CPython is the implementation of Python in C'
.
Summary
- The
\b
represents a word boundary in a string. - Use the
r'\bword\b'
pattern uses the \b to match the wholeword