rg: regex parse error: unclosed character class

The error

> rg 'cfg[' src
rg: regex parse error:
    (?:cfg[)
          ^
error: unclosed character class

With a bracket of the other kind, it's error: unclosed group:

> rg 'log(' src
rg: regex parse error:
    (?:log()
    ^
error: unclosed group

Why it happens

rg treats every pattern as a regular expression. There, [ starts a set of characters and ( starts a group, and each must be closed. Code is full of both. The quieter case is worse: rg 'cfg["items"]' is a valid regex (cfg followed by one of the characters "items), so rg finds nothing and reports no error.

The fix

To search for text exactly as you typed it, add -F (fixed strings):

rg -F 'cfg[' src
rg -F 'log(' src

When you need a regex as well, escape the bracket instead: rg 'cfg\[' src.

In PowerShell, keep the pattern in single quotes so $ and backticks stay as they are. A double quote inside the pattern has its own catch, explained in The string is missing the terminator.