How to compare a char to [ in Julia?

The canonical way would be to use startswith, which works with both single characters and longer strings:

julia> line = "[hello, world]";

julia> startswith(line, '[') # single character
true

julia> startswith(line, "[") # length-1 string
true

julia> startswith(line, "[hello") # longer string
true

If you really want to get the first character of a string it is better to use first since indexing to strings is, in general, tricky.

julia> first(line) == '['
true

See https://docs.julialang.org/en/v1/manual/strings/#Unicode-and-UTF-8-1 for more details about string indexing.

You comparing a string "[" not a char '['

Hope it solve your problem