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+<appendix id="regular-expressions">
+
+<title>Regular Expressions</title>
+
+<synopsis> This Appendix contains a brief but hopefully sufficient and
+covering introduction to the world of <emphasis>regular
+expressions</emphasis>. It documents regular expressions in the form
+available within &kate;, which is not compatible with the regular
+expressions of perl, nor with those of for example
+<command>grep</command>.</synopsis>
+
+<sect1>
+
+<title>Introduction</title>
+
+<para><emphasis>Regular Expressions</emphasis> provides us with a way
+to describe some possible contents of a text string in a way
+understood by a small piece of software, so that it can investigate if
+a text matches, and also in the case of advanced applications with the
+means of saving pieces or the matching text.</para>
+
+<para>An example: Say you want to search a text for paragraphs that
+starts with either of the names <quote>Henrik</quote> or
+<quote>Pernille</quote> followed by some form of the verb
+<quote>say</quote>.</para>
+
+<para>With a normal search, you would start out searching for the
+first name, <quote>Henrik</quote> maybe followed by <quote>sa</quote>
+like this: <userinput>Henrik sa</userinput>, and while looking for
+matches, you would have to discard those not being the beginning of a
+paragraph, as well as those in which the word starting with the
+letters <quote>sa</quote> was not either <quote>says</quote>,
+<quote>said</quote> or so.  And then of cause repeat all of that with
+the next name...</para>
+
+<para>With Regular Expressions, that task could be accomplished with a
+single search, and with a larger degree of preciseness.</para>
+
+<para>To achieve this, Regular Expressions defines rules for
+expressing in details a generalization of a string to match. Our
+example, which we might literally express like this: <quote>A line
+starting with either <quote>Henrik</quote> or <quote>Pernille</quote>
+(possibly following up to 4 blanks or tab characters) followed by a
+whitespace followed by <quote>sa</quote> and then either
+<quote>ys</quote> or <quote>id</quote></quote> could be expressed with
+the following regular expression:</para> <para><userinput>^[
+\t]{0,4}(Henrik|Pernille) sa(ys|id)</userinput></para>
+
+<para>The above example demonstrates all four major concepts of modern
+Regular Expressions, namely:</para>
+
+<itemizedlist>
+<listitem><para>Patterns</para></listitem>
+<listitem><para>Assertions</para></listitem>
+<listitem><para>Quantifiers</para></listitem>
+<listitem><para>Back references</para></listitem>
+</itemizedlist>
+
+<para>The caret (<literal>^</literal>) starting the expression is an
+assertion, being true only if the following matching string is at the
+start of a line.</para>
+
+<para>The stings <literal>[ \t]</literal> and
+<literal>(Henrik|Pernille) sa(ys|id)</literal> are patterns. The first
+one is a <emphasis>character class</emphasis> that matches either a
+blank or a (horizontal) tab character; the other pattern contains
+first a subpattern matching either <literal>Henrik</literal>
+<emphasis>or</emphasis> <literal>Pernille</literal>, then a piece
+matching the exact string <literal> sa</literal> and finally a
+subpattern matching either <literal>ys</literal>
+<emphasis>or</emphasis> <literal>id</literal></para>
+
+<para>The string <literal>{0,4}</literal> is a quantifier saying
+<quote>anywhere from 0 up to 4 of the previous</quote>.</para>
+
+<para>Because regular expression software supporting the concept of
+<emphasis>back references</emphasis> saves the entire matching part of
+the string as well as sub-patterns enclosed in parentheses, given some
+means of access to those references, we could get our hands on either
+the whole match (when searching a text document in an editor with a
+regular expression, that is often marked as selected) or either the
+name found, or the last part of the verb.</para>
+
+<para>All together, the expression will match where we wanted it to,
+and only there.</para>
+
+<para>The following sections will describe in details how to construct
+and use patterns, character classes, assertions, quantifiers and
+back references, and the final section will give a few useful
+examples.</para>
+
+</sect1>
+
+<sect1 id="regex-patterns">
+
+<title>Patterns</title>
+
+<para>Patterns consists of literal strings and character
+classes. Patterns may contain sub-patterns, which are patterns enclosed
+in parentheses.</para>
+
+<sect2>
+<title>Escaping characters</title>
+
+<para>In patterns as well as in character classes, some characters
+have a special meaning.  To literally match any of those characters,
+they must be marked or <emphasis>escaped</emphasis> to let the regular
+expression software know that it should interpret such characters in
+their literal meaning.</para>
+
+<para>This is done by prepending the character with a backslash
+(<literal>\</literal>).</para>
+
+
+<para>The regular expression software will silently ignore escaping a
+character that does not have any special meaning in the context, so
+escaping for example a <quote>j</quote> (<userinput>\j</userinput>) is
+safe. If you are in doubt whether a character could have a special
+meaning, you can therefore escape it safely.</para>
+
+<para>Escaping of cause includes the backslash character it self, to
+literally match a such, you would write
+<userinput>\\</userinput>.</para>
+
+</sect2>
+
+<sect2>
+<title>Character Classes and abbreviations</title>
+
+<para>A <emphasis>character class</emphasis> is an expression that
+matches one of a defined set of characters. In Regular Expressions,
+character classes are defined by putting the legal characters for the
+class in square brackets, <literal>[]</literal>, or by using one of
+the abbreviated classes described below.</para>
+
+<para>Simple character classes just contains one or more literal
+characters, for example <userinput>[abc]</userinput> (matching either
+of the letters <quote>a</quote>, <quote>b</quote> or <quote>c</quote>)
+or <userinput>[0123456789]</userinput> (matching any digit).</para>
+
+<para>Because letters and digits have a logical order, you can
+abbreviate those by specifying ranges of them:
+<userinput>[a-c]</userinput> is equal to <userinput>[abc]</userinput>
+and <userinput>[0-9]</userinput> is equal to
+<userinput>[0123456789]</userinput>.  Combining these constructs, for
+example <userinput>[a-fynot1-38]</userinput> is completely legal (the
+last one would match, of cause, either of
+<quote>a</quote>,<quote>b</quote>,<quote>c</quote>,<quote>d</quote>,
+<quote>e</quote>,<quote>f</quote>,<quote>y</quote>,<quote>n</quote>,<quote>o</quote>,<quote>t</quote>,
+<quote>1</quote>,<quote>2</quote>,<quote>3</quote> or
+<quote>8</quote>).</para>
+
+<para>As capital letters are different characters from their
+non-capital equivalents, to create a caseless character class matching
+<quote>a</quote> or <quote>b</quote>, in any case, you need to write it
+<userinput>[aAbB]</userinput>.</para>
+
+<para>It is of cause possible to create a <quote>negative</quote>
+class matching as <quote>anything but</quote> To do so put a caret
+(<literal>^</literal>) at the beginning of the class: </para>
+
+<para><userinput>[^abc]</userinput> will match any character
+<emphasis>but</emphasis> <quote>a</quote>, <quote>b</quote> or
+<quote>c</quote>.</para>
+
+<para>In addition to literal characters, some abbreviations are
+defined, making life still a bit easier:
+
+<variablelist>
+
+<varlistentry>
+<term><userinput>\a</userinput></term>
+<listitem><para> This matches the <acronym>ASCII</acronym> bell character (BEL, 0x07).</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\f</userinput></term>
+<listitem><para> This matches the <acronym>ASCII</acronym> form feed character (FF, 0x0C).</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\n</userinput></term>
+<listitem><para> This matches the <acronym>ASCII</acronym> line feed character (LF, 0x0A, Unix newline).</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\r</userinput></term>
+<listitem><para> This matches the <acronym>ASCII</acronym> carriage return character (CR, 0x0D).</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\t</userinput></term>
+<listitem><para> This matches the <acronym>ASCII</acronym> horizontal tab character (HT, 0x09).</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\v</userinput></term>
+<listitem><para> This matches the <acronym>ASCII</acronym> vertical tab character (VT, 0x0B).</para></listitem>
+</varlistentry>
+<varlistentry>
+<term><userinput>\xhhhh</userinput></term>
+
+<listitem><para> This matches the Unicode character corresponding to
+the hexadecimal number hhhh (between 0x0000 and 0xFFFF). \0ooo (i.e.,
+\zero ooo) matches the <acronym>ASCII</acronym>/Latin-1 character
+corresponding to the octal number ooo (between 0 and
+0377).</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>.</userinput> (dot)</term>
+<listitem><para> This matches any character (including newline).</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\d</userinput></term>
+<listitem><para> This matches a digit. Equal to <literal>[0-9]</literal></para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\D</userinput></term>
+<listitem><para> This matches a non-digit. Equal to <literal>[^0-9]</literal> or <literal>[^\d]</literal></para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\s</userinput></term>
+<listitem><para> This matches a whitespace character. Practically equal to <literal>[ \t\n\r]</literal></para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\S</userinput></term>
+<listitem><para> This matches a non-whitespace. Practically equal to <literal>[^ \t\r\n]</literal>, and equal to <literal>[^\s]</literal></para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\w</userinput></term>
+<listitem><para>Matches any <quote>word character</quote> - in this case any letter or digit. Note that
+underscore (<literal>_</literal>) is not matched, as is the case with perl regular expressions.
+Equal to <literal>[a-zA-Z0-9]</literal></para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\W</userinput></term>
+<listitem><para>Matches any non-word character - anything but letters or numbers.
+Equal to <literal>[^a-zA-Z0-9]</literal> or <literal>[^\w]</literal></para></listitem>
+</varlistentry>
+
+
+</variablelist>
+
+</para>
+
+<para>The abbreviated classes can be put inside a custom class, for
+example to match a word character, a blank or a dot, you could write
+<userinput>[\w \.]</userinput></para> 
+
+<note> <para>The POSIX notation of classes, <userinput>[:&lt;class
+name&gt;:]</userinput> is currently not supported.</para> </note>
+
+<sect3>
+<title>Characters with special meanings inside character classes</title>
+
+<para>The following characters has a special meaning inside the
+<quote>[]</quote> character class construct, and must be escaped to be
+literally included in a class:</para>
+
+<variablelist>
+<varlistentry>
+<term><userinput>]</userinput></term>
+<listitem><para>Ends the character class. Must be escaped unless it is the very first character in the
+class (may follow an unescaped caret)</para></listitem>
+</varlistentry>
+<varlistentry>
+<term><userinput>^</userinput> (caret)</term>
+<listitem><para>Denotes a negative class, if it is the first character. Must be escaped to match literally if it is the first character in the class.</para></listitem> 
+</varlistentry>
+<varlistentry>
+<term><userinput>-</userinput> (dash)</term>
+<listitem><para>Denotes a logical range. Must always be escaped within a character class.</para></listitem>
+</varlistentry>
+<varlistentry>
+<term><userinput>\</userinput> (backslash)</term>
+<listitem><para>The escape character. Must always be escaped.</para></listitem>
+</varlistentry>
+
+</variablelist>
+
+</sect3>
+
+</sect2>
+
+<sect2>
+
+<title>Alternatives: matching <quote>one of</quote></title>
+
+<para>If you want to match one of a set of alternative patterns, you
+can separate those with <literal>|</literal> (vertical bar character).</para>
+
+<para>For example to find either <quote>John</quote> or <quote>Harry</quote> you would use an expression <userinput>John|Harry</userinput>.</para>
+
+</sect2>
+
+<sect2>
+
+<title>Sub Patterns</title>
+
+<para><emphasis>Sub patterns</emphasis> are patterns enclosed in
+parentheses, and they have several uses in the world of regular
+expressions.</para>
+
+<sect3>
+
+<title>Specifying alternatives</title>
+
+<para>You may use a sub pattern to group a set of alternatives within
+a larger pattern. The alternatives are separated by the character
+<quote>|</quote> (vertical bar).</para>
+
+<para>For example to match either of the words <quote>int</quote>,
+<quote>float</quote> or <quote>double</quote>, you could use the
+pattern <userinput>int|float|double</userinput>. If you only want to
+find one if it is followed by some whitespace and then some letters,
+put the alternatives inside a subpattern:
+<userinput>(int|float|double)\s+\w+</userinput>.</para>
+
+</sect3>
+
+<sect3>
+
+<title>Capturing matching text (back references)</title>
+
+<para>If you want to use a back reference, use a sub pattern to have
+the desired part of the pattern remembered.</para>
+
+<para>For example, it you want to find two occurrences of the same
+word separated by a comma and possibly some whitespace, you could
+write <userinput>(\w+),\s*\1</userinput>. The sub pattern
+<literal>\w+</literal> would find a chunk of word characters, and the
+entire expression would match if those were followed by a comma, 0 or
+more whitespace and then an equal chunk of word characters.  (The
+string <literal>\1</literal> references <emphasis>the first sub pattern
+enclosed in parentheses</emphasis>)</para>
+
+<para>See also <link linkend="backreferences">Back references</link>.</para>
+
+</sect3>
+
+<sect3 id="lookahead-assertions">
+<title>Lookahead Assertions</title>
+
+<para>A lookahead assertion is a sub pattern, starting with either
+<literal>?=</literal> or <literal>?!</literal>.</para>
+
+<para>For example to match the literal string <quote>Bill</quote> but
+only if not followed by <quote> Gates</quote>, you could use this
+expression: <userinput>Bill(?! Gates)</userinput>.  (This would find
+<quote>Bill Clinton</quote> as well as <quote>Billy the kid</quote>,
+but silently ignore the other matches.)</para>
+
+<para>Sub patterns used for assertions are not captured.</para>
+
+<para>See also <link linkend="assertions">Assertions</link></para>
+
+</sect3>
+
+</sect2>
+
+<sect2 id="special-characters-in-patterns">
+<title>Characters with a special meaning inside patterns</title>
+
+<para>The following characters have meaning inside a pattern, and
+must be escaped if you want to literally match them:
+
+<variablelist>
+
+<varlistentry>
+<term><userinput>\</userinput> (backslash)</term>
+<listitem><para>The escape character.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>^</userinput> (caret)</term>
+<listitem><para>Asserts the beginning of the string.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>$</userinput></term>
+<listitem><para>Asserts the end of string.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>()</userinput> (left and right parentheses)</term>
+<listitem><para>Denotes sub patterns.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>{}</userinput> (left and right curly braces)</term>
+<listitem><para>Denotes numeric quantifiers.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>[]</userinput> (left and right square brackets)</term>
+<listitem><para>Denotes character classes.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>|</userinput> (vertical bar)</term>
+<listitem><para>logical OR. Separates alternatives.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>+</userinput> (plus sign)</term>
+<listitem><para>Quantifier, 1 or more.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>*</userinput> (asterisk)</term>
+<listitem><para>Quantifier, 0 or more.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>?</userinput> (question mark)</term>
+<listitem><para>An optional character. Can be interpreted as a quantifier, 0 or 1.</para></listitem>
+</varlistentry>
+
+</variablelist>
+
+</para>
+
+</sect2>
+
+</sect1>
+
+<sect1 id="quantifiers">
+<title>Quantifiers</title>
+
+<para><emphasis>Quantifiers</emphasis> allows a regular expression to
+match a specified number or range of numbers of either a character,
+character class or sub pattern.</para>
+
+<para>Quantifiers are enclosed in curly brackets (<literal>{</literal>
+and <literal>}</literal>) and have the general form
+<literal>{[minimum-occurrences][,[maximum-occurrences]]}</literal>
+</para>
+
+<para>The usage is best explained by example:
+
+<variablelist>
+
+<varlistentry>
+<term><userinput>{1}</userinput></term>
+<listitem><para>Exactly 1 occurrence</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>{0,1}</userinput></term>
+<listitem><para>Zero or 1 occurrences</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>{,1}</userinput></term>
+<listitem><para>The same, with less work;)</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>{5,10}</userinput></term>
+<listitem><para>At least 5 but maximum 10 occurrences.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>{5,}</userinput></term>
+<listitem><para>At least 5 occurrences, no maximum.</para></listitem>
+</varlistentry>
+
+</variablelist>
+
+</para>
+
+<para>Additionally, there are some abbreviations:
+
+<variablelist>
+
+<varlistentry>
+<term><userinput>*</userinput> (asterisk)</term>
+<listitem><para>similar to <literal>{0,}</literal>, find any number of occurrences.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>+</userinput> (plus sign)</term>
+<listitem><para>similar to <literal>{1,}</literal>, at least 1 occurrence.</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>?</userinput> (question mark)</term>
+<listitem><para>similar to <literal>{0,1}</literal>, zero or 1 occurrence.</para></listitem>
+</varlistentry>
+
+</variablelist>
+
+</para>
+
+<sect2>
+
+<title>Greed</title>
+
+<para>When using quantifiers with no maximum, regular expressions
+defaults to match as much of the searched string as possible, commonly
+known as <emphasis>greedy</emphasis> behavior.</para>
+
+<para>Modern regular expression software provides the means of
+<quote>turning off greediness</quote>, though in a graphical
+environment it is up to the interface to provide you with access to
+this feature. For example a search dialog providing a regular
+expression search could have a check box labeled <quote>Minimal
+matching</quote> as well as it ought to indicate if greediness is the
+default behavior.</para>
+
+</sect2>
+
+<sect2>
+<title>In context examples</title>
+
+<para>Here are a few examples of using quantifiers</para>
+
+<variablelist>
+
+<varlistentry>
+<term><userinput>^\d{4,5}\s</userinput></term>
+<listitem><para>Matches the digits in <quote>1234 go</quote> and <quote>12345 now</quote>, but neither in <quote>567 eleven</quote>
+nor in <quote>223459 somewhere</quote></para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\s+</userinput></term>
+<listitem><para>Matches one or more whitespace characters</para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>(bla){1,}</userinput></term>
+<listitem><para>Matches all of <quote>blablabla</quote> and the <quote>bla</quote> in <quote>blackbird</quote> or <quote>tabla</quote></para></listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>/?&gt;</userinput></term>
+<listitem><para>Matches <quote>/&gt;</quote> in <quote>&lt;closeditem/&gt;</quote> as well as
+<quote>&gt;</quote> in <quote>&lt;openitem&gt;</quote>.</para></listitem>
+</varlistentry>
+
+</variablelist>
+
+</sect2>
+
+</sect1>
+
+<sect1 id="assertions">
+<title>Assertions</title>
+
+<para><emphasis>Assertions</emphasis> allows a regular expression to
+match only under certain controlled conditions.</para>
+
+<para>An assertion does not need a character to match, it rather
+investigates the surroundings of a possible match before acknowledging
+it. For example the <emphasis>word boundary</emphasis> assertion does
+not try to find a non word character opposite a word one at its
+position, instead it makes sure that there is not a word
+character. This means that the assertion can match where there is no
+character, i.e. at the ends of a searched string.</para>
+
+<para>Some assertions actually does have a pattern to match, but the
+part of the string matching that will not be a part of the result of
+the match of the full expression.</para>
+
+<para>Regular Expressions as documented here supports the following
+assertions:
+
+<variablelist>
+
+<varlistentry> 
+<term><userinput>^</userinput> (caret: beginning of
+string)</term> 
+<listitem><para>Matches the beginning of the searched
+string.</para> <para>The expression <userinput>^Peter</userinput> will
+match at <quote>Peter</quote> in the string <quote>Peter, hey!</quote>
+but not in <quote>Hey, Peter!</quote> </para> </listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>$</userinput> (end of string)</term>
+<listitem><para>Matches the end of the searched string.</para>
+
+<para>The expression <userinput>you\?$</userinput> will match at the
+last you in the string <quote>You didn't do that, did you?</quote> but
+nowhere in <quote>You didn't do that, right?</quote></para>
+
+</listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\b</userinput> (word boundary)</term>
+<listitem><para>Matches if there is a word character at one side and not a word character at the
+other.</para>
+<para>This is useful to find word ends, for example both ends to find
+a whole word. The expression <userinput>\bin\b</userinput> will match
+at the separate <quote>in</quote> in the string <quote>He came in
+through the window</quote>, but not at the <quote>in</quote> in
+<quote>window</quote>.</para></listitem>
+
+</varlistentry>
+
+<varlistentry>
+<term><userinput>\B</userinput> (non word boundary)</term>
+<listitem><para>Matches wherever <quote>\b</quote> does not.</para>
+<para>That means that it will match for example within words: The expression
+<userinput>\Bin\B</userinput> will match at in <quote>window</quote> but not in <quote>integer</quote> or <quote>I'm in love</quote>.</para>
+</listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>(?=PATTERN)</userinput> (Positive lookahead)</term>
+<listitem><para>A lookahead assertion looks at the part of the string following a possible match.
+The positive lookahead will prevent the string from matching if the text following the possible match
+does not match the <emphasis>PATTERN</emphasis> of the assertion, but the text matched by that will
+not be included in the result.</para>
+<para>The expression <userinput>handy(?=\w)</userinput> will match at <quote>handy</quote> in
+<quote>handyman</quote> but not in <quote>That came in handy!</quote></para>
+</listitem>
+</varlistentry>
+
+<varlistentry>
+<term><userinput>(?!PATTERN)</userinput> (Negative lookahead)</term>
+
+<listitem><para>The negative lookahead prevents a possible match to be
+acknowledged if the following part of the searched string does match
+its <emphasis>PATTERN</emphasis>.</para>
+<para>The expression <userinput>const \w+\b(?!\s*&amp;)</userinput>
+will match at <quote>const char</quote> in the string <quote>const
+char* foo</quote> while it can not match <quote>const QString</quote>
+in <quote>const QString&amp; bar</quote> because the
+<quote>&amp;</quote> matches the negative lookahead assertion
+pattern.</para>
+</listitem>
+</varlistentry>
+
+</variablelist>
+
+</para>
+
+</sect1>
+
+<sect1 id="backreferences">
+
+<title>Back References</title>
+
+<para></para>
+
+</sect1>
+
+</appendix>