ASCII

Standard ASCII is a 7-bit character set defined in 1963 (ANSI X3.4). The first 128 codepoints. Modern Unicode (UTF-8) is fully backward- compatible with ASCII, every ASCII byte represents the same character in UTF-8.

Control Characters (0-31, 127)

Non-printing control codes. Most are historical; a handful still matter in 2026 (TAB, LF, CR, BEL, ESC, BS, DEL).

Dec / Oct	Hex	Char	C-esc Description	-
0 / 000	00	NUL	\0 null	-
1 / 001	01	SOH	-	start of heading
2 / 002	02	STX	-	start of text
3 / 003	03	ETX	-	end of text (Ctrl-C interrupt)
4 / 004	04	EOT	-	end of transmission (Ctrl-D EOF)
5 / 005	05	ENQ	-	enquiry
6 / 006	06	ACK	-	acknowledgment
7 / 007	07	BEL	\a bell / alert	-
8 / 010	08	BS	\b backspace	-
9 / 011	09	HT	\t horizontal tab	-
10 / 012	0A	LF	\n line feed (Unix newline)	-
11 / 013	0B	VT	\v vertical tab	-
12 / 014	0C	FF	\f form feed	-
13 / 015	0D	CR	\r carriage return (Mac-classic newline)	-
14 / 016	0E	SO	-	shift out
15 / 017	0F	SI	-	shift in
16 / 020	10	DLE	-	data link escape
17 / 021	11	DC1	-	device control 1 (Ctrl-Q / XON)
18 / 022	12	DC2	-	device control 2
19 / 023	13	DC3	-	device control 3 (Ctrl-S / XOFF)
20 / 024	14	DC4	-	device control 4
21 / 025	15	NAK	-	negative ack
22 / 026	16	SYN	-	synchronous idle
23 / 027	17	ETB	-	end transmission block
24 / 030	18	CAN	-	cancel
25 / 031	19	EM	-	end of medium
26 / 032	1A	SUB	-	substitute (Ctrl-Z suspend on Unix)
27 / 033	1B	ESC	\e escape (terminal escape sequences)	-
28 / 034	1C	FS	-	file separator
29 / 035	1D	GS	-	group separator
30 / 036	1E	RS	-	record separator
31 / 037	1F	US	-	unit separator
127 / 177	7F	DEL	-	delete (often the Backspace key)

CRLF = \r\n (DOS / HTTP / SMTP); LF = \n (Unix); CR = \r (classic Mac).

Printable Characters (32-126)

Space and visible characters, the 7-bit ASCII subset every operator decodes in their head. Knowing the ranges (digits start at 48, uppercase at 65, lowercase at 97) is what makes byte dumps in xxd or hexdump -C readable without a lookup.

Dec / Oct	Hex	Char	Notes
32 / 040	20	SPC	space
33 / 041	21	!	-
34 / 042	22	“	double quote
35 / 043	23	#	-
36 / 044	24	$	-
37 / 045	25	%	-
38 / 046	26	&	-
39 / 047	27	‘	apostrophe / single quote
40 / 050	28	(	-
41 / 051	29	)	-
42 / 052	2A		-
43 / 053	2B		-
44 / 054	2C	,	-
45 / 055	2D		-
46 / 056	2E	.	-
47 / 057	2F	/	-
48-57 / 060-71 30-39 0-9	-	-	digits
58 / 072	3A	:	-
59 / 073	3B	;	-
60 / 074	3C	<	-
61 / 075	3D	=	-
62 / 076	3E	>	-
63 / 077	3F	?	-
64 / 100	40	@	-
65-90 / 101-132 41-5A A-Z	-	-	uppercase letters
91 / 133	5B	[	-
92 / 134	5C	\	backslash
93 / 135	5D	]	-
94 / 136	5E	^	-
95 / 137	5F	_	-
96 / 140 97-122 / 141-172 61-7A a-z lowercase letters	60	`	backtick
123 / 173	7B	{	-
124 / 174	7C		-
125 / 175	7D	}	-
126 / 176	7E	~	-

Letter / Digit Codepoint Tricks

Useful relationships an experienced operator just remembers. Most are about the regular spacing of the alphabet ranges, once you know that case differs by a single bit (0x20), you can convert between cases with bitops without reaching for a library function.

• Digits '0' to '9' are 48 to 57 (0x30 to 0x39).

• Uppercase 'A' to 'Z' are 65 to 90 (0x41 to 0x5A).

• Lowercase 'a' to 'z' are 97 to 122 (0x61 to 0x7A).

• The difference between an uppercase and lowercase letter is exactly 0x20 (32). 'A' | 0x20 == 'a'; 'a' & ~0x20 == 'A'.

• Digit-to-int: '0' + n gives the character; c - '0' gives the value.

• 'A' + (n % 26) cycles through the alphabet.

Escape Sequences

Common escape sequences and what they map to. Most of these are identical across Bash, C, Python, and JavaScript string literals – \n is always newline, \t is always tab, \xHH is always a byte by hex.

Sequence	Code (dec)	Description
\0	0	null
\a	7	bell
\b	8	backspace
\t	9	horizontal tab
\n	10	line feed (newline)
\v	11	vertical tab
\f	12	form feed
\r	13	carriage return
\e	27	escape (GNU extension; \033 is portable)
\\	92	backslash
\'	39	single quote
\"	34	double quote
\xHH	hex	any byte by hex (\x41 = ‘A’)
\nnn	octal	any byte by octal (\101 = ‘A’)
\uHHHH	Unicode	BMP code point (é = é)
\UHHHHHHHH Unicode	-	full code point (\U0001F600 = 😀)

Terminal ESC Sequences

Almost all “ANSI escape codes” begin with ESC (27, \033 , \x1b, \e) followed by [ (a “Control Sequence Introducer”). The most common ones:

Sequence	Effect
\e[0m	reset all attributes
\e[1m	bold
\e[2m	dim
\e[3m	italic
\e[4m	underline
\e[7m	reverse
\e[30-37m	foreground (black, red, green, yellow, blue, magenta, cyan, white)
\e[40-47m	background (same colors)
\e[90-97m \e[100-107m bright background \e[38;5;Nm 256-color foreground	bright foreground
\e[38;2;R;G;Bm	truecolor foreground
\e[2J	clear screen
\e[H	cursor home
\e[N;Mf	cursor position (row N, column M)
\e[?25l/h	hide / show cursor

Example:

$ printf '\e[31;1mError:\e[0m something failed\n'

ASCII vs. Unicode

The encoding context every operator should keep straight. ASCII is the 7-bit common ancestor; Latin-1 and Windows-1252 are 8-bit extensions still alive in legacy data; UTF-8 is the modern universal default and a strict superset of ASCII at the byte level.

• ASCII is 7-bit (codepoints 0-127). Every ASCII byte is the same byte in UTF-8.

• Latin-1 / ISO-8859-1 extended ASCII to 8 bits with European characters.

• Windows-1252 extended Latin-1 with curly quotes, em-dash, etc. Often misidentified as Latin-1.

• UTF-8 is the universal encoding in 2026; ASCII is a strict subset.

• Code page 437 is the original IBM PC character set with the classic line-drawing characters; appears in BIOS / DOS / serial- console contexts.

For modern programming, treat strings as Unicode (UTF-8) and use ASCII only when you mean specifically the 0-127 range.

Looking It Up

The on-machine lookups when you need a codepoint in the moment. man ascii is everywhere; ord() and chr() are the Python equivalents; xxd shows you the byte values of any file or string.

In a shell:

$ man ascii
$ ascii 65
$ echo -n A | xxd

In Python:

ord('A')              # 65
chr(65)               # 'A'
'A'.encode('utf-8')   # b'A'
bytes([65, 66, 67])   # b'ABC'

In JavaScript:

'A'.charCodeAt(0)     // 65
String.fromCharCode(65) // 'A'