Add preliminary regexp module documentation.

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+# Rspamd regexp module
+
+This is a core module that deals with regexp expressions to filter messages.
+
+## Principles of work
+
+Regexp module operates with `expressions` - a logical sequence of different `atoms`. Atoms
+are elements of the expression and could be represented as regular expressions, rspamd
+functions and lua functions. Rspamd supports the following operators in expressions:
+
+* `&&` - logical AND (can be also written as `and` or even `&`)
+* `||` - logical OR (`or` `|`)
+* `!` - logical NOT (`not`)
+* `+` - logical PLUS, usually used with comparisions:
+       - `>` more than
+       - `<` less than
+       - `>=` more or equal
+       - `<=` less or equal
+
+Whilst logical operators are clear for understanding, PLUS is not so clear. In rspamd,
+it is used to join multiple atoms or subexpressions and compare them to a specific number:
+
+       A + B + C + D > 2 - evaluates to `true` if at least 3 operands are true
+       (A & B) + C + D + E >= 2 -  evaluates to `true` if at least 2 operands are true
+
+Operators has their own priorities:
+       
+1. NOT
+2. PLUS
+3. COMPARE
+4. AND
+5. OR
+
+You can change priorities by braces, of course. All operations are *right* associative in rspamd.
+While evaluating expressions, rspamd tries to optimize their execution time by reordering and does not evaluate
+unnecessary branches.
+
+## Expressions components
+
+Rspamd support the following components within expressions:
+
+* Regular expressions
+* Internal functions
+* Lua global functions (not widely used)
+
+### Regular expressions
+
+In rspamd, regular expressions could match different parts of messages:
+
+* Headers (should be `Header-Name=/regexp/flags`)
+* Textual mime parts
+* Raw messages
+* URLs
+
+The match type is defined by special flags after the last `/` symbol:
+
+* `H` - header regexp
+* `M` - raw message regexp
+* `P` - part regexp
+* `U` - URL regexp
+
+We strongly discourage from using of raw message regexps as they are expensive and
+should be replaced by [trie](trie.md) rules if possible.
+
+Each regexp also supports the following flags:
+
+* `i` - ignore case
+* `u` - use utf8 regexp
+* `m` - multiline regexp
+* `x` - extended regexp
+* `O` - do not optimize regexp (rspamd optimizes regexps by default)
+
+### Internal functions
+
+Rspamd supports a set of internal functions to do some common spam filtering tasks:
+
+* `check_smtp_data(type[, str or /re/])` - checks for the specific envelope argument: `from`, `rcpt`, `user`, `subject`
+* `compare_encoding(str or /re/)` - compares message encoding with string or regexp
+* `compare_parts_distance(inequality_percent)` - if a message is multipart/alternative, compare two parts and return `true` if they are inequal more than `inequality_percent`
+* `compare_recipients_distance(inequality_percent)` - check how different are recipients of a message (works for > 5 recipients)
+* `compare_transfer_encoding(str or /re/)` - compares message transfer encoding with string or regexp
+* `content_type_compare_param(param, str or /re/)` - compare content-type parameter `param` with string or regexp
+* `content_type_has_param(param)` - return true if `param` exists in content-type
+* `content_type_is_subtype(str or /re/` - return `true` if subtype of content-type matches string or regexp
+* `content_type_is_type(str or /re/)`- return `true` if type of content-type matches string or regexp
+* `has_content_part(type)` - return `true` if the part with the specified `type` exists
+* `has_content_part_len(type, len)` - return `true` if the part with the specified `type` exists and have at least `len` lenght
+* `has_fake_html()` - check if there is an HTML part in message with no HTML tags
+* `has_html_tag(tagname)` - return `true` if html part contains specified tag
+* `has_only_html_part()` - return `true` if there is merely a single HTML part
+* `header_exists(header)` - return if a specified header exists in the message
+* `is_html_balanced()` - check whether HTML part has balanced tags
+* `is_recipients_sorted()` - return `true` if there are more than 5 recipients in a message and they are sorted
+* `raw_header_exists()` - does the same as `header_exists`
+
+Many of these functions are just legacy but they are supported in terms of compatibility.
+
+### Lua atoms
+
+Lua atoms now can be lua global functions names. This is supported merely for compatibility and it is 
+a subject of future redesign.
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