A PrintableString is a Universal Tag

Tag (metadata)

In online computer systems terminology, a tag is a non-hierarchical keyword or term assigned to a piece of information . This kind of metadata helps describe an item and allows it to be found again by browsing or searching...

 in the ASN.1

Abstract Syntax Notation One

Data generated at various sources of observation need to be transmitted to one or more locations that process it to generate useful results. For example, voluminous signal data collected by a radio telescope from outer space. The system recording the data and the system processing it later may be...

 notation.

According to RFC 3280 http://www.ietf.org/rfc/rfc3280.txt, PrintableString is an acceptable character set for the DirectoryName data type. PrintableString comprises a subset

Subset

In mathematics, especially in set theory, a set A is a subset of a set B if A is "contained" inside B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment...

 of the ASCII

ASCII

The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...

 character set, and does not include the at sign

At sign

The at sign , also called the ampersat, apetail, arroba, atmark, at symbol, commercial at or monkey tail, is formally an abbreviation of the accounting and commercial invoice term "at the rate of"...

 (@) or ampersand

Ampersand

An ampersand is a logogram representing the conjunction word "and". The symbol is a ligature of the letters in et, Latin for "and".-Etymology:...

 (&). This sometimes causes problems for naive implementers who attempt to put an SMTP email address into an X.509

X.509

In cryptography, X.509 is an ITU-T standard for a public key infrastructure and Privilege Management Infrastructure . X.509 specifies, amongst other things, standard formats for public key certificates, certificate revocation lists, attribute certificates, and a certification path validation...

 digital certificate Distinguished Name.

According to the Layman's Guide to a Subset of ASN.1, BER, and DER http://www.columbia.edu/~ariel/ssleay/layman.html, the character set of PrintableString can be expressed as:

A, B, ..., Z

a, b, ..., z

0, 1, ..., 9

(space) ' + , - . / : = ?