Python | Exploring the behavior of greedy and nongreedy matching in pattern matching

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Greedy vs. nongreedy matching

In regular expressions, the quantifiers *, +, and ? are greedy by default, which means that they will match as much as possible. However, you can also use the *?, +?, and ?? syntax to make them non-greedy, which means that they will match as little as possible.

Here's an example to illustrate the difference between greedy and non-greedy matching:

import re

text = "<html><head><title>Title</title></head><body>Body</body></html>"

pattern1 = "<.*>" # Greedy match

pattern2 = "<.*?>" # Non-greedy match

matches1 = re.findall(pattern1, text)

matches2 = re.findall(pattern2, text)

print("Greedy matches:", matches1)

print("Non-greedy matches:", matches2)

In this example, we are trying to match the HTML tags in the given text using two different patterns. The first pattern is a greedy match, which uses the * quantifier to match any number of characters between the < and > characters. The second pattern is a non-greedy match, which uses the *? quantifier to match as few characters as possible between the < and > characters.

The output of this program is:

Greedy matches: ['<html><head><title>Title</title></head><body>Body</body></html>']

Non-greedy matches: ['<html>', '<head>', '<title>', '</title>', '</head>', '<body>', '</body>', '</html>']

As you can see, the greedy match matches the entire HTML document, while the non-greedy match matches each individual HTML tag separately.

You can use the non-greedy matching syntax with any quantifier, as well as with other regular expression syntax such as character classes, grouping with parentheses, and alternation with the | operator.