Python | Exploring the power of positive look-ahead for advanced pattern matching

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Positive look-ahead

Positive look-ahead is a type of lookaround construct in regular expressions that matches a pattern only if it is followed by another pattern, without including the second pattern in the match.

In Python's re module, the syntax for positive look-ahead is (?=pattern), where pattern is the pattern that must follow the match. For example, the regular expression foo(?=bar) would match the string "foo" only if it is followed by the string "bar".

Here's an example:

import re

text = "The foo is followed by bar, but not by baz."

pattern = r"foo(?=bar)"

matches = re.findall(pattern, text)

print(matches)

In this example, the regular expression foo(?=bar) matches the string "foo" only if it is followed by the string "bar". The re.findall() function finds all non-overlapping matches of the pattern in the input string text.

The output of this program is:

['foo']

Notice that the regular expression matches only the first occurrence of "foo", because it is followed by "bar".

Positive look-ahead constructs are useful when you want to match a pattern only if it is followed by another pattern, without including that pattern in the match. They can be very powerful tools for building complex regular expressions.