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<title>Regular Expressions Tutorial</title>
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<h4>Found at: http://publish.ez.no/article/articleprint/11/</h3>
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<h1>Regular Expressions Explained</h1>
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<p class="byline">Author: Jan Borsodi</p>
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<p class="byline">Publishing date: 30.10.2000 18:02</p>
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<p>
This article will give you an introduction to the world of <i>regular
expressions</i>. I'll start off with explaining what regular expressions are
and introduce their syntax, then some examples with varying complexity.
<p>
<h2>Concept</h2>
A <i>regular expression</i> is a text pattern consisting of a combination of
alphanumeric characters and special characters known as metacharacters. A
close relative is in fact the <i>wildcard expression</i> which are often used
in file management. The pattern is used to match against text strings. The
result of a match is either successful or not, however when a match is
successful not all of the pattern must match, this is explained later in the
article.
<p>
You'll find that <i>regular expressions</i> are used in three different ways:
Regular text match, search and replace and splitting. The latter is basicly the
same as the reverse match <i>i.e.</i> everything the <i>regular expression</i>
did not match.
<p>
<i>Regular expressions</i> are often simply called regexps or RE, but for
consistency I'll be referring to it with its full name.
<p>
Due to the versatility of the <i>regular expression</i> it is widely used in
text processing and parsing. UNIX users are probably familiar with them through
the use of the programs, <i>grep</i>, <i>sed</i>, <i>awk</i> and <i>ed</i>.
Text editors such as <i>(X)Emacs</i> and <i>vi</i> also use them heavily.
Probably the most known use of <i>regular expressions</i> are in the
programming language Perl; you'll find that Perl sports the most advanced
<i>regular expression</i> implementation to this day.
<p>
<h2>Usage</h2>
Now you're probably wondering why you should bother to learn <i>regular
expressions</i>. Well if you're a normal computer user your benefits from
using them are somewhat small, however if you're either a developer or a system
administrator you'll find that knowing <i>regular expressions</i> will make
your (professional) life so much better.
<p>
Developers can use them to parse text files, fix up code and other wonders.
System administrators can use them to search through logs, automate boring
tasks and sniff the network traffic for unauthorized activity.
<p>
Actually I would go so far as to say it's a crime for a System Administrator
not to have <b>any</b> knowledge of <i>regular expressions</i>.
<p>
<h2>Quantifiers</h2>
The contents of an expression are, as explained earlier, a combination of
alphanumeric characters and metacharacters. An alphanumeric character is either
a letter from the alphabet:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>abc</code>
</td>
</tr>
</table>
<p>
or a number:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>123</code>
</td>
</tr>
</table>
<p>
Actually in the world of regular expressions any character which is not a
metacharacter will match itself (often called literal characters), however a
lot of the time you're mostly concerned with the alphanumeric characters. A
very special character is the backslash <b>\</b>, as this turns any
metacharacters into literal characters, and alphanumeric characters into a
sort of metacharacter or sequence. The metacharacters are:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>\ | ( ) [ { ^ $ * + ? . < ></code>
</td>
</tr>
</table>
<p>
With that said normal characters don't sound too interesting so let's jump to
our very first metacharacters.
<p>
The punctuation mark, or dot, <b>.</b> needs explaining first since it often
leads to confusion. This character will not, as many might think, match the
punctuation in a line. It is instead a special metacharacter which matches any
character. Using this where you wanted to find the end of the line, or the
decimal in a floating number, will lead to strange results. As explained above,
you need to backslashify it to get the literal meaning. For instance take this
expression:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>1.23</code>
</td>
</tr>
</table>
<p>
will match the number 1.23 in a text as you might have guessed, but it will
<b>also</b> match these next lines:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>1x23</code><br>
<code>1 23</code><br>
<code>1-23</code>
</td>
</tr>
</table>
<p>
to make the expression <b>only</b> match the floating number we change it to:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>1\.23</code>
</td>
</tr>
</table>
<p>
Remember this, it's very important. Now with that said we can get the show
going.
<p>
Two heavily recurring metacharacters are:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>*</code> and <code>+</code>
</td>
</tr>
</table>
<p>
They are called quantifiers and tell the engine to look for several occurrences
of a character; the quantifier always precedes the character at hand. The
<code><b>*</b></code> character matches <b>zero or more</b> occurrences of the
character in a row, the <code><b>+</b></code> character is similar but matches
<b><i>one</i> or more</b>.
<p>
So if you decided to find words which had the character <code><b>c</b></code>
in it you might be tempted to write:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>c*</code>
</td>
</tr>
</table>
<p>
What might come as a surprise to you is that you will find an enormous amount
of matches, even words with no <code><b>c</b></code> in them will match.
"How is this so?" you ask. Well, the answer is simple. Recall that the
<code><b>*</b></code> character matches <b>zero</b> or more characters, and
that's exactly what you matched, zero characters.
<p>
You see, in <i>regular expressions</i> you have the possibility to match what
is called the <b>empty string</b>, which is simply a string with zero size.
This empty string can actually be found in all texts. For instance the word:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>go</code>
</td>
</tr>
</table>
<p>
contains three empty strings. They are contained at the position right before
the <code><b>g</b></code>, in between the <code><b>g</b></code> and the
<code><b>o</b></code> and after the <code><b>o</b></code>. And an empty string
contains exactly <b>one</b> empty string. At first this might seem like a
really silly thing to do, but you'll learn later on how this is used in more
complex expressions.
<p>
So with this knowledge we might want to change our expression to:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>c+</code>
</td>
</tr>
</table>
<p>
and voila we get only words with <code><b>c</b></code> in them.
<p>
The next metacharacter you'll learn is:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>?</code>
</td>
</tr>
</table>
<p>
This simply tells the engine to either match the character or not (zero or
one). For instance the expression:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>cows?</code>
</td>
</tr>
</table>
<p>
will match any of these lines:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>cow</code><br>
<code>cows</code>
</td>
</tr>
</table>
<p>
These three metacharacters are simply a specialized scenario for the more
generalized quantifier:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>{n,m}</code>
</td>
</tr>
</table>
<p>
the <code><b>n</b></code> and <code><b>m</b></code> are respectively the
minimum and maximum size for the quantifier. For instance:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>{1,5}</code>
</td>
</tr>
</table>
<p>
means match one or up to five characters. You can also skip
<code><b>m</b></code> to allow for infinite match:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>{1,}</code>
</td>
</tr>
</table>
<p>
which matches one or more characters. This is exactly what the
<code><b>+</b></code> character does. So now you see the connection,
<code><b>*</b></code> is equal to <code><b>{0,}</b></code>,
<code><b>+</b></code> is equal to <code><b>{1,}</b></code> and
<code><b>?</b></code> is equal to <code><b>{0,1}</b></code>.
<p>
The last thing you can do with the quantifier is to also skip the comma:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>{5}</code>
</td>
</tr>
</table>
<p>
which means to match 5 characters, no more, no less.
<p>
<h2>Assertions</h2>
The next type of metacharacters is assertions. These will match if a given
assertion is true. The first pair of assertions are:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>^</code> and <code>$</code>
</td>
</tr>
</table>
<p>
which match the beginning of the line, and the end of the line, respectively.
Note that some <i>regular expression</i> implementations allows you to change
their behavior so that they will instead match the beginning of the text and
the end of the text. These assertions always match a zero length string, or in
other words, they match a position. For instance, if you wrote this expression:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>^The</code>
</td>
</tr>
</table>
<p>
it would match any line which began with the word <code><b>The</b></code>.
<p>
The next assertion characters match at the beginning and end of a word; they
are:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code><</code> and <code>></code>
</td>
</tr>
</table>
<p>
they come in handy when you want to match a word precisely. For instance:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>cow</code>
</td>
</tr>
</table>
<p>
would match any of the following words:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>cow</code><br>
<code>coward</code><br>
<code>cowage</code><br>
<code>cowboy</code><br>
<code>cowl</code>
</td>
</tr>
</table>
<p>
A small change to the expression:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code><cow></code>
</td>
</tr>
</table>
<p>
and you'll only match the word <code><b>cow</b></code> in the text.
<p>
One last thing to be said is that all literal characters are in fact assertions
themselves. The difference between them and the ones above is that literal
characters have a size. So for cleanliness sake we only use the word
"assertions" for those that are zero-width.
<p>
<h2>Groups and Alternation</h2>
One thing you might have noticed when we explained quantifiers is that they
only worked on the character to the left. Since this pretty much limits our
expressions I'll explain other uses for quantifiers. Quantifiers can also be
used on metacharacters; using them on assertions is silly since assertions are
zero-width and matching one, two, three or more of them doesn't do any good.
However the grouping and sequence metacharacters are perfect for being
quantified. Let's first start with grouping.
<p>
You can form groups, or subexpressions as they are frequently called, by using
the begin and end parenthesis characters:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>(</code> and <code>)</code>
</td>
</tr>
</table>
<p>
The <code><b>(</b></code> starts the subexpression and the <code><b>)</b></code>
ends it. It is also possible to have one or more subexpressions inside a
subexpressions. The subexpression will match if the contents match. So mixing
this with quantifiers and assertions you can do:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>( ?ho)+</code>
</td>
</tr>
</table>
<p>
which matches all of the following lines:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>ho</code><br>
<code>ho ho</code><br>
<code>ho ho ho</code><br>
<code>hohoho</code>
</td>
</tr>
</table>
<p>
Another use for subexpressions are to extract a portion of the match if it
matches. This is often used in conjunction with sequences, which are discussed
later.
<p>
You can also use the result of a subexpression for what is called a back
reference. A back reference is given by using a backslashified digit, a single
non-zero digit. This leaves you with nine back references (0 through 9). The
back reference matches whatever the corresponding subexpression actually
matched (except that <code>{<i>article_contents_1</i>}</code> matches a null
character). To find the number of the subexpression, count the open parentheses
from the left.
<p>
The uses for back references are somewhat limited, especially since you only
have nine of them, but on some rare occasion you might need it. Note some
<i>regular expression</i> implementations can use multi-digit numbers as long
as they don't start with a 0.
<p>
Next are alternations, which allow you to match on any of many words. The
alternation character is:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>|</code>
</td>
</tr>
</table>
<p>
A sample usage is:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>Bill|Linus|Steve|Larry</code>
</td>
</tr>
</table>
<p>
would match either Bill, Linus, Steve or Larry. Mixing this with subexpressions
and quantifiers we can do:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>cow(ard|age|boy|l)?</code>
</td>
</tr>
</table>
<p>
which matches any of the following words but no others:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>cow</code><br>
<code>coward</code><br>
<code>cowage</code><br>
<code>cowboy</code><br>
<code>cowl</code>
</td>
</tr>
</table>
<p>
I mentioned earlier in the article that not all of the expression must match
for the match to be successful. This can happen when you're using
subexpressions together with alternations. For example:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>((Donald|Dolly) Duck)|(Scrooge McDuck)</code>
</td>
</tr>
</table>
<p>
As you see only the left or right top subexpression will match, not both. This
is sometimes handy when you want to run a complex pattern in one subexpression
and if it fails try another one.
<p>
<h2>Sequences</h2>
Last we have sequences, which define sequences of characters which can match.
Sometimes you don't want to match a word directly but rather something that
resembles one. The sequence characters are:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>[</code> and <code>]</code>
</td>
</tr>
</table>
<p>
Any characters put inside the sequence brackets are treated as literal
characters, even metacharacters. The only special characters are
<code><b>-</b></code> which denotes character ranges, and <code><b>^</b></code>
which is used to negate a sequence. The sequence is somewhat similar to
alternation; the similarity is that only one of the items listed will match.
For instance:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>[a-z]</code>
</td>
</tr>
</table>
<p>
will match any lowercase characters which are in the English alphabet (a to z).
Another common sequence is:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>[a-zA-Z0-9]</code>
</td>
</tr>
</table>
<p>
Which matches any lowercase or capital characters in the English alphabet as
well as numbers. Sequences are also mixed with quantifiers and assertions to
produce more elaborate searches. Example:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code><[a-zA-Z]+></code>
</td>
</tr>
</table>
<p>
matches all whole words. This will match:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>cow</code><br>
<code>Linus</code><br>
<code>regular</code><br>
<code>expression</code>
</td>
</tr>
</table>
<p>
but will not match:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>200</code><br>
<code>x-files</code><br>
<code>C++</code>
</td>
</tr>
</table>
<p>
Now if you wanted to find anything <i>but</i> words, the expression:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>[^a-zA-Z0-9]+</code>
</td>
</tr>
</table>
<p>
would find any sequences of characters which do not contain the English
alphabet or any numbers.
<p>
Some implementations of <i>regular expressions</i> allow you to use shorthand
versions for commonly used sequences, they are:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>\d<code>, a digit (<code>[0-9]</code>)<br>
<code>\D</code>, a non-digit (<code>[^0-9]</code>)<br>
<code>\w</code>, a word (alphanumeric) (<code>[a-zA-Z0-9]</code>)<br>
<code>\W</code>, a non-word (<code>[^a-zA-Z0-9]</code>)<br>
<code>\s</code>, a whitespace (<code>[ \t\n\r\f]</code>)<br>
<code>\S</code>, a non-whitespace (<code>[^ \t\n\r\f]</code>)
</td>
</tr>
</table>
<p>
<b>Wildcards</b><br>
For people who have some knowledge with wildcards (also known as
<i>file globs</i> or <i>file globbing</i>), I'll give a brief explanation on how
to convert them to <i>regular expressions</i>. After reading this article, you
probably have seen the similarities with wildcards. For instance:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>*.jpg</code>
</td>
</tr>
</table>
<p>
matches any text which end with <code><b>.jpg</b></code>. You can also specify
brackets with characters, as in:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>*.[ch]pp</code>
</td>
</tr>
</table>
<p>
matches any text which ends in <code><b>.cpp</b></code> or
<code><b>.hpp</b></code>. Altogether very similar to regular expressions.
<p>
The <code><b>*</b></code> means match zero or more of anything in wildcards. As
we learned, we do this is regular expression with the punctuation mark and the
<code><b>*</b></code> quantifier. This gives:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>.*</code>
</td>
</tr>
</table>
<p>
Also remember to convert any punctuation marks from wildcards to be
backslashified.
<p>
The <code><b>?</b></code> means match any character but do match
<b>something</b>. This is exactly what the punctuation mark does.
<p>
Square brackets can be used untouched since they have the same meaning going
from wildcards to regular expressions.
<p>
These leaves us with:
<ul>
<li>Replace any <code><b>*</b></code> characters with <code><b>.*</b></code>
<li>Replace any <code><b>?</b></code> characters with <code><b>.</b></code>
<li>Leave square brackets as they are.
<li>Replace any characters which are metacharacters with a backslashified
version.
</ul>
<p>
<b>Examples</b><br>
<p>
<table width=100% border="0">
<tr>
<td bgcolor="#f0f0f0">
<code>*.jpg</code>
</td>
</tr>
</table>
<p>
would be converted to:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>.*\.jpg</code>
</td>
</tr>
</table>
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>ez*.[ch]pp</code>
</td>
</tr>
</table>
<p>
would convert to:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>ez.*\.[ch]pp</code>
</td>
</tr>
</table>
<p>
or alternatively:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>ez.*\.(cpp|hpp)</code>
</td>
</tr>
</table>
<p>
<h3>Example Regular Expressions</h3>
To really get to know <i>regular expressions</i> I've left some commonly used
expressions on this page. Study them, experiment and try to understand exactly
what they are doing.
<p>
Email validity: will only match email addresses which are valid, such as
"user@host.com":
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>[a-z0-9_-]+(\.[a-z0-9_-]+)*@[a-z0-9_-]+(\.[a-z0-9_-]+)+</code>
</td>
</tr>
</table>
<p>
Email validity #2: matches email addresses with a name in front, like "John
Doe <user@host.com>":
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>("?[a-zA-Z]+"?[ \t]*)+\<[a-z0-9_-]+(\.[a-z0-9_-]+)*@[a-z0-9_-]+(\.[a-z0-9_-]+)+\></code>
</td>
</tr>
</table>
<p>
Protocol validity: matches web based protocols such as "htpp://",
"ftp://" or "https://":
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>[a-z]+://</code>
</td>
</tr>
</table>
<p>
C/C++ includes: matches valid include statements in C/C++ files:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>^#include[ \t]+[<"][^>"]+[">]</code>
</td>
</tr>
</table>
<p>
C++ end of line comments:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>//.+$</code>
</td>
</tr>
</table>
<p>
C/C++ span line comments (it has one flaw, can you spot it?):
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>/\*[^*]*\*/</code>
</td>
</tr>
</table>
<p>
Floating point numbers: matches simple floating point numbers of the kind 1.2
and 0.5:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>-?[0-9]+\.[0-9]+</code>
</td>
</tr>
</table>
<p>
Hexadecimal numbers: matches C/C++ style hex numbers, <i>e.g.</i>
<code>0xcafebabe</code>:
<p>
<table width=100% border=0>
<tr>
<td bgcolor="#f0f0f0">
<code>0x[0-9a-fA-F]+</code>
</td>
</tr>
</table>
</body>
</html>
