ch10.htm

this is a book on pearl , simple example with explanation is given here. it could be beneficial for

BBB&quot;</TT>, then <TT>m/\w+/;</TT>

would match the <TT>&quot;AAA&quot;</TT>

in the string. If <TT>$_</TT> was

blank, full of white space, or full of other non-word characters,

an undefined value would be returned.

<P>

The preceding pattern will let you determine if <TT>$_</TT>

contains a word but does not let you know what the word is. In

order to accomplish that, you need to eNClose the matching components

inside parentheses. For example:

<BLOCKQUOTE>

<PRE>

m/(\w+)/;

</PRE>

</BLOCKQUOTE>

<P>

By doing this, you force Perl to store the matched string into

the $1 variable. The $1 variable can be considered as pattern

memory.

<P>

This introduction to pattern components describes most of the

details you need to know in order to create your own patterns

or regular expressions. However, some of the components deserve

a bit more study. The next few sections look at character classes,

quantifiers, pattern memory, pattern precedeNCe, and the extension

syntax. Then the rest of the chapter is devoted to showing specific

examples of when to use the different components.

<H3><A NAME="ExampleCharacterClasses">

Example: Character Classes</A></H3>

<P>

A character class defines a type of character. The character class

<TT>[0123456789]</TT> defines the

class of decimal digits, and <TT>[0-9a-f]</TT>

defines the class of hexadecimal digits. Notice that you can use

a dash to define a range of consecutive characters. Character

classes let you match any of a range of characters; you don't

know in advaNCe which character will be matched. This capability

to match non-specific characters is what meta-characters are all

about.

<P>

You can use variable interpolation inside the character class,

but you must be careful when doing so. For example,

<BLOCKQUOTE>

<PRE>

$_ = &quot;AAABBBccC&quot;;

$charList = &quot;ADE&quot;;

print &quot;matched&quot; if m/[$charList]/;

</PRE>

</BLOCKQUOTE>

<P>

will display

<BLOCKQUOTE>

<PRE>

matched

</PRE>

</BLOCKQUOTE>

<P>

This is because the variable interpolation results in a character

class of <TT>[ADE]</TT>. If you use

the variable as one-half of a character range, you need to ensure

that you don't mix numbers and digits. For example,

<BLOCKQUOTE>

<PRE>

$_ = &quot;AAABBBccC&quot;;

$charList = &quot;ADE&quot;;

print &quot;matched&quot; if m/[$charList-9]/;

</PRE>

</BLOCKQUOTE>

<P>

will result in the following error message when executed:

<BLOCKQUOTE>

<PRE>

/[ADE-9]/: invalid [] range in regexp at test.pl line 4.

</PRE>

</BLOCKQUOTE>

<P>

At times, it's necessary to match on any character except for

a given character list. This is done by complementing the character

class with the caret. For example,

<BLOCKQUOTE>

<PRE>

$_ = &quot;AAABBBccC&quot;;

print &quot;matched&quot; if m/[^ABC]/;

</PRE>

</BLOCKQUOTE>

<P>

will display nothing. This match returns true only if a character

besides <TT>A</TT>, <TT>B</TT>,

or <TT>C</TT> is in the searched string.

If you complement a list with just the letter <TT>A</TT>,

<BLOCKQUOTE>

<PRE>

$_ = &quot;AAABBBccC&quot;;

print &quot;matched&quot; if m/[^A]/;

</PRE>

</BLOCKQUOTE>

<P>

then the string <TT>&quot;matched&quot;</TT>

will be displayed because <TT>B</TT>

and <TT>C</TT> are part of the string-in

other words, a character besides the letter <TT>A</TT>.

<P>

Perl has shortcuts for some character classes that are frequently

used. Here is a list of what I call symbolic character classes:

<BR>



<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD WIDTH=67><CENTER><TT><B><FONT FACE="Courier">\w</FONT></B></TT></CENTER>

</TD><TD WIDTH=523>This symbol matches any alphanumeric character or the underscore character. It is equivalent to the character class <TT>[a-zA-Z0-9_]</TT>.

</TD></TR>

<TR><TD WIDTH=67><CENTER><TT><B><FONT FACE="Courier">\W</FONT></B></TT></CENTER>

</TD><TD WIDTH=523>This symbol matches every character that the <TT>\w</TT> symbol does not. In other words, it is the complement of <TT>\w</TT>. It is equivalent to <TT>[^a-zA-Z0-9_]</TT>.

</TD></TR>

<TR><TD WIDTH=67><CENTER><TT><B><FONT FACE="Courier">\s</FONT></B></TT></CENTER>

</TD><TD WIDTH=523>This symbol matches any space, tab, or newline character. It is equivalent to <TT>[\t \n]</TT>.

</TD></TR>

<TR><TD WIDTH=67><CENTER><TT><B><FONT FACE="Courier">\S</FONT></B></TT></CENTER>

</TD><TD WIDTH=523>This symbol matches any non-whitespace character. It is equivalent to <TT>[^\t \n]</TT>.

</TD></TR>

<TR><TD WIDTH=67><CENTER><TT><B><FONT FACE="Courier">\d</FONT></B></TT></CENTER>

</TD><TD WIDTH=523>This symbol matches any digit. It is equivalent to <TT>[0-9]</TT>.

</TD></TR>

<TR><TD WIDTH=67><CENTER><TT><B><FONT FACE="Courier">\D</FONT></B></TT></CENTER>

</TD><TD WIDTH=523>This symbol matches any non-digit character. It is equivalent to <TT>[^0-9]</TT>.

</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

You can use these symbols inside other character classes, but

not as endpoints of a range. For example, you can do the following:

<BLOCKQUOTE>

<PRE>

$_ = &quot;\tAAA&quot;;

print &quot;matched&quot; if m/[\d\s]/;

</PRE>

</BLOCKQUOTE>

<P>

which will display

<BLOCKQUOTE>

<PRE>

matched

</PRE>

</BLOCKQUOTE>

<P>

because the value of <TT>$_</TT> iNCludes

the tab character.<BR>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD><B>Tip</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

Meta-characters that appear inside the square brackets that define a character class are used in their literal sense. They lose their meta-meaning. This may be a little confusing at first. In fact, I have a tendeNCy to forget this when evaluating 
patterns.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD><B>Note</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

I think that most of the confusion regarding regular expressions lies in the fact that each character of a pattern might have several possible meanings. The caret could be an aNChor, it could be a caret, or it could be used to complement a character 
class. Therefore, it is vital that you decide which context any given pattern character or symbol is in before assigning a meaning to it.</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<H3><A NAME="ExampleQuantifiers">

Example: Quantifiers</A></H3>

<P>

Perl provides several different quantifiers that let you specify

how many times a given component must be present before the match

is true. They are used when you don't know in advaNCe how many

characters need to be matched. Table 10.6 lists the different

quantifiers that can be used.<BR>

<P>

<CENTER><B>Table 10.6&nbsp;&nbsp;The Six Types of Quantifiers</B></CENTER>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD WIDTH=91><CENTER><I>Quantifier</I></CENTER></TD><TD WIDTH=492><I>Description</I>

</TD></TR>

<TR><TD WIDTH=91><CENTER>*</CENTER></TD><TD WIDTH=492>The component must be present zero or more times.

</TD></TR>

<TR><TD WIDTH=91><CENTER>+</CENTER></TD><TD WIDTH=492>The component must be present one or more times.

</TD></TR>

<TR><TD WIDTH=91><CENTER>?</CENTER></TD><TD WIDTH=492>The component must be present zero or one times.

</TD></TR>

<TR><TD WIDTH=91><CENTER>{n}</CENTER></TD><TD WIDTH=492>The component must be present n times.

</TD></TR>

<TR><TD WIDTH=91><CENTER>{n,}</CENTER></TD><TD WIDTH=492>The component must be present at least n times.

</TD></TR>

<TR><TD WIDTH=91><CENTER>{n,m}</CENTER></TD><TD WIDTH=492>The component must be present at least n times and no more than m times.

</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

If you need to match a word whose length is unknown, you need

to use the <TT>+</TT> quantifier.

You can't use an <TT>*</TT> because

a zero length word makes no sense. So, the match statement might

look like this:

<BLOCKQUOTE>

<PRE>

m/\w+/;

</PRE>

</BLOCKQUOTE>

<P>

This pattern will match <TT>&quot;QQQ&quot;</TT>

and <TT>&quot;AAAAA&quot;</TT> but

not <TT>&quot;&quot;</TT> or <TT>&quot;

 BBB&quot;</TT>. In order to account for the leading white

space, which may or may not be at the beginning of a string, you

need to use the asterisk (<TT>*</TT>)

quantifier in conjuNCtion with the <TT>\s</TT>

symbolic character class in the following way:

<BLOCKQUOTE>

<PRE>

m/\s*\w+/; <BR>



</PRE>

</BLOCKQUOTE>

<p>

<CENTER>

<TABLE BORDERCOLOR=#000000 BORDER=1 WIDTH=80%>

<TR><TD><B>Tip</B></TD></TR>

<TR><TD>

<BLOCKQUOTE>

Be careful when using the <TT>*</TT> quantifier because it can match an empty string, which might not be your intention. The pattern <TT>/b*/</TT> will match any string-even one without any <TT>b</TT> characters.

</BLOCKQUOTE>



</TD></TR>

</TABLE>

</CENTER>

<P>

<P>

At times, you may need to match an exact number of components.

The following match statement will be true only if five words

are present in the <TT>$_</TT> variable:

<BLOCKQUOTE>

<PRE>

$_ = &quot;AA AB AC AD AE&quot;;

m/(\w+\s+){5}/;

</PRE>

</BLOCKQUOTE>

<P>

In this example, we are matching at least one word character followed

by zero or more white space characters. The <TT>{5}</TT>

quantifier is used to ensure that that combination of components

is present five times.

<P>

The <TT>*</TT> and <TT>+</TT>

quantifiers are greedy. They match as many characters as possible.

This may not always be the behavior that you need. You can create

non-greedy components by following the quantifier with a <TT>?</TT>.

<P>

Use the following file specification in order to look at the <TT>*</TT>

and <TT>+</TT> quantifiers more closely:

<BLOCKQUOTE>

<PRE>

$_ = '/user/Jackie/temp/names.dat';

</PRE>

</BLOCKQUOTE>

<P>

The regular expression <TT>.*</TT>

will match the entire file specification. This can be seen in

the following small program:

<BLOCKQUOTE>

<PRE>

$_ = '/user/Jackie/temp/names.dat';

m/.*/;

print $&amp;;

</PRE>

</BLOCKQUOTE>

<P>

This program displays

<BLOCKQUOTE>

<PRE>

/user/Jackie/temp/names.dat

</PRE>

</BLOCKQUOTE>

<P>

You can see that the <TT>*</TT> quantifier

is greedy. It matched the whole string. If you add the ? modifier

to make the <TT>.*</TT> component

non-greedy, what do you think the program would display?

<BLOCKQUOTE>

<PRE>

$_ = '/user/Jackie/temp/names.dat';

m/.*?/;

print $&amp;;

</PRE>

</BLOCKQUOTE>

<P>

This program displays nothing because the least amount of characters

that the <TT>*</TT> matches is zero.

If we change the <TT>*</TT> to a <TT>+</TT>,

then the program will display

<BLOCKQUOTE>

<PRE>

/

</PRE>

</BLOCKQUOTE>

<P>

Next, let's look at the coNCept of pattern memory, which lets

you keep bits of matched string around after the match is complete.

<H3><A NAME="ExamplePatternMemory">

Example: Pattern Memory</A></H3>

<P>

Matching arbitrary numbers of characters is fine, but without

the capability to find out what was matched, patterns would not

be very useful. Perl lets you eNClose pattern components inside

parentheses in order to store the string that matched the components

into pattern memory. You also might hear <I>pattern memory </I>referred

to as <I>pattern buffers</I>. This memory persists after the match

statement is finished executing so that you can assign the matched

values to other variables.

<P>

You saw a simple example of this earlier right after the component

descriptions. That example looked for the first word in a string

and stored it into the first buffer, <TT>$1</TT>.

The following small program

<BLOCKQUOTE>

<PRE>

$_ =  &quot;AAA BBB ccC&quot;;

m/(\w+)/;

print(&quot;$1\n&quot;);