SyntaxError: Invalid regular expression: Invalid group

How browsers word it

Common causes

1. A Python named group

   Python writes (?P<year>...). JavaScript has no P in a named group.

   Input

   (?P<year>\d{4})

   Regex Tester says

   Invalid regular expression: /(?P<year>\d{4})/: Invalid group

2. A group name that starts with a digit

   A group name must start with a letter or an underscore.

   Input

   (?<1a>x)

   Regex Tester says

   Invalid regular expression: /(?<1a>x)/: Invalid capture group name

3. The same name used twice

   Each named group needs its own name within one pattern.

   Input

   (?<n>a)(?<n>b)

   Regex Tester says

   Invalid regular expression: /(?<n>a)(?<n>b)/: Duplicate capture group name

4. The JavaScript form works

   Without the P, the same group is valid.

   Input

   (?<year>\d{4})

   Regex Tester says

   Compiles and matches

How to fix it

1. Write named groups as (?<name>...), and back-references as \k<name>.
2. Start every group name with a letter or underscore, and use each name once.
3. When you port a pattern from Python, PCRE or Java, check the group syntax first.
4. Paste the pattern into the regex tester to see the exact error.