Does Your Language Support Regex Lookbehind?

Support at a glance

Engine	Lookbehind?	Variable-length?	Notes
JavaScript	Yes (ES2018)	Yes	(?<=\$)\d+ works in modern V8/SpiderMonkey
Python re	Yes	No — fixed width only	Use the PyPI regex module for variable-length
Java	Yes	Bounded	Allows bounded quantifiers like (?<=a{1,3})
PHP / PCRE	Yes	No (PCRE1); \K workaround	\K resets the match start as a lookbehind alternative
Go (RE2)	No	—	RE2 rejects lookaround by design (linear-time guarantee)

Use it in your code

# Python: grab the number after a dollar sign, without capturing the $
import re
re.findall(r'(?<=\$)\d+', 'Price $42 and $7')   # -> ['42', '7']

// JavaScript (ES2018+)
'Price $42 and $7'.match(/(?<=\$)\d+/g);          // -> ['42', '7']

The variable-length trap

Python's built-in re requires a fixed-width lookbehind: (?<=abc) is fine, (?<=a+) raises error: look-behind requires fixed-width pattern. If you need variable-length lookbehind in Python, install the third-party regex module, which supports it. JavaScript and .NET allow variable-length lookbehind natively.

No lookbehind? Two escapes

• Capture and drop: match what precedes with a group and reference group 1 instead of the whole match — e.g. \$(\d+) and read group 1.
• PCRE \K: \$\K\d+ matches the $ then resets the reported match start, so only the digits are returned.