Between delimiters

(?<=prefix)pattern(?=suffix)

JS:  'price: [42] and size: [XL]'.match(/(?<=\[)\d+(?=\])/g)
Py:  re.findall(r'(?<=\[)\w+(?=\])', '[hello] and [world]')

JS: ["42"]  (digits inside square brackets, without brackets)
Py: ["hello", "world"]

Note You can combine multiple lookarounds on the same position. A common pattern for password validation stacks multiple lookaheads: (?=.*[A-Z])(?=.*\d)(?=.*[@#$]).{8,} checks for uppercase, digit, and special character all in one pass.

.*?target  (scan forward minimally until target is found)

JS:  'START data1 END noise START data2 END'.match(/START(.*?)END/g)
Py:  re.findall(r'START(.*?)END', 'START data1 END noise START data2 END')

JS: ["START data1 END", "START data2 END"]
Py: [" data1 ", " data2 "]  (findall returns groups)

Note The .*? idiom scans forward character by character until the following pattern matches. This is the standard approach for extracting content between delimiters. Be aware that .*? can still cause slowdowns on long strings with no match -- consider using negated character classes instead when possible.