Flavor Differences — Where Lookaround Breaks

Lookaround is the most fragmented feature in regex

You'd think "look around the position" would be universal. It isn't. Here's the actual landscape as of 2026:

Engine	Lookahead	Lookbehind	Variable-width Lookbehind
Python `re`	✓	✓	✗ (raises error)
Python `regex`	✓	✓	✓
JavaScript	✓	✓ (ES2018)	✓ (ES2018)
PCRE	✓	✓	Partial (depends on version)
Java	✓	✓	Limited (max length must be bounded)
.NET	✓	✓	✓
Go `regexp` / RE2	✗	✗	✗
ripgrep (default)	✗	✗	✗
ripgrep `--pcre2`	✓	✓	Partial

The portability spectrum

Universal: No lookaround. Use anchors and character classes only.

Most engines: Lookahead (positive and negative). Almost always supported.

Most engines, but quirky: Lookbehind, fixed-width. Python, Java, .NET. Watch the width rule.

Modern engines only: Variable-width lookbehind. JavaScript ES2018+, .NET, Python regex.

Never: Go, RE2. Patterns using lookaround simply won't compile.

Practical advice

If your code needs to run in multiple languages or your regex might end up in a Go service, avoid lookaround. Use anchors, captures, and post-processing. The pattern is uglier but portable.

If you're in one engine and stay there, use lookaround freely — it's the cleanest tool for many problems.

Code

Same problem, different engines·text

# Goal: extract numbers preceded by '$' but skip the '$' itself

# Python re — works
import re
re.findall(r'(?<=\$)\d+', 'paid $19 not 20 but $5')
# ['19', '5']

# JavaScript ES2018+ — works
// 'paid $19 not 20 but $5'.match(/(?<=\$)\d+/g)
// ['19', '5']

# Go — does NOT work, regexp.MustCompile panics
// regexp.MustCompile(`(?<=\$)\d+`)  // panic: error parsing regexp

# Workaround for Go — capture and post-process
// re := regexp.MustCompile(`\$(\d+)`)
// matches := re.FindAllStringSubmatch(s, -1)
// for _, m := range matches { fmt.Println(m[1]) }  // m[1] is just the number

Flavor Differences — Where Lookaround Breaks

Lookaround is the most fragmented feature in regex

The portability spectrum

Practical advice

Code

External links

Exercise

Progress

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