Lesson 42 of 55
10 mins readPython Lookarounds, Word Boundaries & Anchors
In Plain English
Zero-width assertions match positions without consuming characters: anchors (`^`, `$`, `\b`, `\A`, `\z`), lookaheads (`(?=...)`, `(?!...)`), and lookbehinds (`(?<=...)`, `(?<!...)`).
Deep Dive: How It Works
String Anchors: `\A` (beginning of string), `\z` (absolute end of string).
Word Boundary (\b): Matches transition between word `\w` and non-word characters.
Positive Lookahead (?=...): Matches only if followed by the subpattern.
Positive Lookbehind (?<=...): Matches only if preceded by the subpattern.
Core Rules to Remember
Zero-Width Assertions: Validate surrounding context without including it in the match.
Precise Word Boundaries: `\bcat\b` matches "cat" but ignores "caterpillar" or "bobcat".
Live Interactive Example
Hit Run Code to see it liveExtracting Prices with Lookbehind
Python 3.12
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Output Console
Click "Run Code" to view the rendered output.
How it works: Lookbehind extracted 49.99 without the dollar sign, and \b matched the exact word "cat".
Your Turn: Micro Challenge
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Micro Exercise
Match Exact Keyword with Word Boundaries
- Set `my $str = "perl scripting in perl5";`
- Extract all exact occurrences of word `\bperl\b` into `my @matches = ($str =~ /\bperl\b/g);`
- Print `"Matches: " . scalar(@matches) . "\n"`.
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Sandbox Output
Click "Run & Check" to test your solution.
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