Note: This applies to plain Ruby scripts, Rails does not have this issue.

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When you work with Ruby strings, those strings will get some default encoding, depending on how they are created. Most strings get the encoding Encoding.default_internal or UTF-8, if no encoding is set. This is the default and just fine.

However, some strings will instead get Encoding.default_external, notably

• the string inside a StringIO.new
• some strings created via CSV
• files read from disk
• strings read from an IRB

Encoding.default_external defaults to whatever locale charmap says on your system. This is usually UTF-8 as well, but can default to something less sane.

If you encounter mysterious encoding errors (like Encoding::CompatibilityError: incompatible character encodings: ISO-8859-1 and UTF-8) this might be what happened.

You can override this behaviour by manually setting Encoding.default_external = 'UTF-8'. You should do this at the very beginning of your code.