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diff --git a/src/lithium_spf_lexer.xrl b/src/lithium_spf_lexer.xrl
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+++ b/src/lithium_spf_lexer.xrl
@@ -0,0 +1,90 @@
+% SPDX-FileCopyrightText: 2025 Johannes Christ
+% SPDX-License-Identifier: AGPL-3.0-or-later
+
+Definitions.
+
+ALPHA = [a-zA-Z]
+WHITESPACE = \s
+% 4.6.1
+QUALIFIER = \+|-|\?|~
+% TODO: technically macro spec
+DOMAIN_SPEC = [^\s]+
+
+QNUM = ([0-9]|([1-9][0-9])|(1[0-9][0-9])|(2[0-4][0-9])|(25[0-5]))
+IP4_NETWORK = {QNUM}\.{QNUM}\.{QNUM}\.{QNUM}
+IP4_CIDR_LENGTH = /([0-9]|([1-2][0-9])|(3[0-2]))
+% XXX: the following causes "cannot match because a previous clause always matches"
+% maybe consider unifying these two cidr lengths
+IP6_CIDR_LENGTH = /[0-9]|([1-9][0-9])|(1[0-1][0-9])|(12[0-8])
+% parsing is done by erlang, I'm not writing a regex for that
+IP6_NETWORK = [a-zA-Z0-9:]+
+MECHANISM_WITH_IP4_NET = ip4:{IP4_NETWORK}({IP4_CIDR_LENGTH})?
+MECHANISM_WITH_IP6_NET = ip6:{IP6_NETWORK}({IP6_CIDR_LENGTH})?
+MECHANISM_NETWORK = {MECHANISM_WITH_IP4_NET}|{MECHANISM_WITH_IP6_NET}
+MECHANISM_WITH_OPTIONAL_DOMAIN = ptr(:{DOMAIN_SPEC})?
+MECHANISM_WITH_MANDATORY_DOMAIN = (exists|redirect|include):{DOMAIN_SPEC}
+MECHANISM_WITH_DOMAIN_AND_CIDR = (a|mx)(:{DOMAIN_SPEC}({DUAL_CIDR_LENGTH})?)?
+MECHANISM_DOMAIN = {MECHANISM_WITH_MANDATORY_DOMAIN}|{MECHANISM_WITH_OPTIONAL_DOMAIN}|{MECHANISM_WITH_DOMAIN_AND_CIDR}
+MECHANISM = (all|{MECHANISM_NETWORK}|{MECHANISM_DOMAIN})
+MECHANISM_TRAILER = :|/
+NAME = [a-zA-Z][a-zA-Z0-9_.-]*
+KNOWN_MODIFIER = (exp|redirect)={DOMAIN_SPEC}
+UNKNOWN_MODIFIER = {NAME}={DOMAIN_SPEC}
+MACRO_STRING = {DOMAIN_SPEC}
+
+% TODO: 4.6.1: mechanism and modifier names are case-insensitive - WHAT THE FUCK!
+
+Rules.
+
+({QUALIFIER})?{MECHANISM} : {token, {directive, mechanism(TokenChars)}}.
+% {QUALIFIER}?{MECHANISM} : {token, {directive, mechanism(TokenChars)}}.
+{KNOWN_MODIFIER} : {token, {modifier, modifier(TokenChars)}}.
+% {MECHANISM} : {token, {mechanism, TokenChars}}.
+% {NAME} : {token, {name, TokenChars}}.
+% = : {token, equal}.
+{WHITESPACE}+ : skip_token.
+% where "name" is not any known modifier
+{UNKNOWN_MODIFIER} : {token, {unknown_modifier, modifier(TokenChars)}}.
+
+
+Erlang code.
+
+split_by(Value, Char) ->
+    [Head | Tail] = string:split(Value, Char),
+    case Tail of
+        "" -> {Head, ""};
+        Rest -> {Head, lists:flatten(lists:join(Char, Rest))}
+    end.
+
+split_name(Value) -> split_by(Value, ":").
+
+split_domain_spec(Value) ->
+    [Head | Tail] = string:split(Value, ":"),
+    {Head, lists:join(Tail, ":")}.
+
+parse_ip_maybe_cidr(Value) ->
+    case string:split(Value, "/") of
+        [IP, RawMask] ->
+            {ok, Address} = inet:parse_address(IP),
+            {Mask, []} = string:to_integer(RawMask),
+            {Address, Mask};
+        [IP] ->
+            {ok, Address} = inet:parse_address(IP),
+            Address
+    end.
+
+parse_mechanism("ip4:" ++ Ip4MaybeCidr) -> parse_ip_maybe_cidr(Ip4MaybeCidr);
+parse_mechanism("ip6:" ++ Ip6MaybeCidr) -> parse_ip_maybe_cidr(Ip6MaybeCidr);
+parse_mechanism(Value) -> split_by(Value, ":").
+
+% 4.6.2
+mechanism("+" ++ Value) -> {pass, parse_mechanism(Value)};
+mechanism("-" ++ Value) -> {fail, parse_mechanism(Value)};
+mechanism("~" ++ Value) -> {softfail, parse_mechanism(Value)};
+mechanism("?" ++ Value) -> {neutral, parse_mechanism(Value)};
+mechanism(Value) -> {pass, parse_mechanism(Value)}.
+
+modifier(Value) -> split_by(Value, "=").
+
+
+% vim: ft=erlang: