Mastering Greedy vs. Non‑Greedy Regex in Python: A Hands‑On Guide
This article explains the difference between greedy and non‑greedy patterns in Python regular expressions, demonstrates their behavior with practical code examples, shows how to capture HTML tags, and introduces named group techniques, helping readers master regex matching nuances.
Introduction
A fan asked about the difference between greedy and non‑greedy modes in Python regular expressions, sparking a lively discussion. This article shares the question and provides a detailed answer.
Solution Process
The answer includes a sample code that demonstrates both greedy and non‑greedy matching.
import re
txt = "This is an HTML tag: <head>HEADER</head>. It means the head of the whole HTML document."
pattern1 = re.compile(r"<.*>")
pattern2 = re.compile(r"<.*?>")
result1 = re.findall(pattern1, txt)
result2 = re.findall(pattern2, txt)
print(result1)
print(result2)The output is shown below.
The two patterns differ only by a ? character, but the effect is significant. The greedy pattern <.*> matches the longest possible string, while the non‑greedy pattern <.*?> stops at the first closing bracket.
pattern1 = re.compile(r"<.*>")
pattern2 = re.compile(r"<.*?>")Visual analysis of the results is illustrated below.
The greedy mode continues matching until the last possible closing tag, whereas the non‑greedy mode stops at the first closing tag.
Bonus
A bonus example shows how to use named groups to capture tag names.
import re
txt = "This is an HTML tag: <head>HEADER</head>. It means the head of the whole HTML document."
tag = re.compile(r"<(?P<tag_mark>[A-Za-z0-9]+)>.*?</(?P=tag_mark)>.*")
print(re.findall(tag, txt))Conclusion
The article answers the fan’s question by clearly explaining greedy vs. non‑greedy regex behavior, providing code samples, output screenshots, and a named‑group technique, thereby helping readers solve similar regex challenges.
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