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Control-character lines are used to display characters whose code values fall outside the range of printable characters from US ASCII (Ecma-6 / ISO 646). That is, those characters whose code values fall below below 32 decimal (those from the C0 set of control characters from Ecma-48 / ISO/IEC 6429); and those whose values are at or above 127 decimal (the “delete” character, and character byte values with the high bit set).
Control-character lines begin with an initial dot, ‘.’, followed by the control characters or high-value bytes being represented.
. BEL/^G NUL/^ CR/^M LF/^J DEL/^? xA0 xFF
Control characters (whose numeric codes fall below decimal 32, plus the delete character at decimal 127) are represented by a mnemonic acronym identifying the character's function. Unless the -C was given, this acronym is followed by a slash, and the control-key combination that would produce the corresponding character (control characters are usually much more recognizable from one or the other of their name or their control-character representation, than they are by their hexadecimal code value). The “control-key combination” representation consists of a “hat” or “circumflex accent” character, followed by a character with a value in the range of 63 through 95 decimal.
Note that the delete character, designated as ‘DEL/^?’, is a special case, in that one can not generally reproduce that key by holding down the control key and typing a question mark; it is simply used as an identification of the key.
Other values (high-value bytes) are represented by the lowercase letter ‘x’ followed by the two-digit hexadecimal code value for the character.
For reference, here's a table of the control characters (plus del). It is based on the information from Table 1 of Ecma-48 / ISO/IEC 6429 (the control-key representation has been added).
| Hex | Key | Name | Hex | Key | Name
|
|---|---|---|---|---|---|
| x00 | ^@ | NUL | x10 | ^P | DLE
|
| x01 | ^A | SOH | x11 | ^Q | DC1
|
| x02 | ^B | STX | x12 | ^R | DC2
|
| x03 | ^C | ETX | x13 | ^S | DC3
|
| x04 | ^D | EOT | x14 | ^T | DC4
|
| x05 | ^E | ENQ | x15 | ^U | NAK
|
| x06 | ^F | ACK | x16 | ^V | SYN
|
| x07 | ^G | BEL | x17 | ^W | ETB
|
| x08 | ^H | BS | x18 | ^X | CAN
|
| x09 | ^I | TAB | x19 | ^Y | EM
|
| x0A | ^J | LF | x1A | ^Z | SUB
|
| x0B | ^K | VT | x1B | ^[ | ESC
|
| x0C | ^L | FF | x1C | ^\ | IS4
|
| x0D | ^M | CR | x1D | ^] | IS3
|
| x0E | ^N | SO | x1E | ^^ | IS2
|
| x0F | ^O | SI | x1F | ^_ | IS1
|
| x7F | ^? | DEL |
|