Regex to find if there is only one block of code

Question

My input is a string, I want to validate that there is only one first-level block of code.

Examples :

{ abc }              TRUE
{ a { bc }         


        

Answer 1

EDIT: I started writing the answer before JavaScript was specified. Will leave it as for the record as it fully explains the regex.

In short: In JavaScript I cannot think of a reliable solution. In other engines there are several options:

• Recursion (on which I will expand below)
• Balancing group (.NET)

For solutions 2 (which anyhow won't work in JS either), I'll refer you to the example in this question

Recursive Regex

In Perl, PCRE (e.g. Notepad++, PHP, R) and the Matthew Barnett's regex module for Python, you can use:

^({(?:[^{}]++|(?1))*})$

The idea is to match exactly one set of nested braces. Anything more makes the regex fail.

See what matches and fails in the Regex Demo.

Explanation

• The ^ anchor asserts that we are at the beginning of the string
• The outer parentheses define Group 1 (or Subroutine 1)
• { match the opening brace
• (?: ... )* zero or more times, we will...
• [^{}]++ match any chars that are not { or }
• OR |
• (?1) repeat the expression of subroutine 1
• } match closing brace
• The $ anchor asserts that we are at the end of the string. Therefore,