Characters allowed in a URL

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EDIT: As @Jukka K. Korpela correctly points out, RFC 1738 was updated by RFC 3986.
This has expanded and clarified the characters valid for host, unfortunately it’s not easily copied and pasted, but I’ll do my best.

In first matched order:

host        = IP-literal / IPv4address / reg-name

IP-literal  = "[" ( IPv6address / IPvFuture  ) "]"

IPvFuture   = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )

IPv6address =         6( h16 ":" ) ls32
                  /                       "::" 5( h16 ":" ) ls32
                  / [               h16 ] "::" 4( h16 ":" ) ls32
                  / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
                  / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
                  / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
                  / [ *4( h16 ":" ) h16 ] "::"              ls32
                  / [ *5( h16 ":" ) h16 ] "::"              h16
                  / [ *6( h16 ":" ) h16 ] "::"

ls32        = ( h16 ":" h16 ) / IPv4address
                  ; least-significant 32 bits of address

h16         = 1*4HEXDIG 
               ; 16 bits of address represented in hexadecimal

IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet

dec-octet   = DIGIT                 ; 0-9
              / %x31-39 DIGIT         ; 10-99
              / "1" 2DIGIT            ; 100-199
              / "2" %x30-34 DIGIT     ; 200-249
              / "25" %x30-35          ; 250-255

reg-name    = *( unreserved / pct-encoded / sub-delims )

unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"     <---This seems like a practical shortcut, most closely resembling original answer

reserved    = gen-delims / sub-delims

gen-delims  = ":" / "/" / "?" / "#" / "[" / "]" / "@"

sub-delims  = "!" / "$" / "&" / "'" / "(" / ")"
              / "*" / "+" / "," / ";" / "="

pct-encoded = "%" HEXDIG HEXDIG

Original answer from RFC 1738 specification:

Thus, only alphanumerics, the special characters “$-_.+!*'(),“, and
reserved characters used for their reserved purposes may be used
unencoded within a URL.

^ obsolete since 1998.

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