ch26_04.htm

the unix power tools

><P
CLASS="para"
>was the first word on a line,</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>the second letter was a lowercase letter,</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>was three letters long (followed by a space character 
(<IMG
SRC="../chars/squ.gif"
ALT=" ">)), and</P
></LI
><LI
CLASS="listitem"
><P
CLASS="para"
>the third letter was a lowercase vowel,</P
></LI
></UL
><P
CLASS="para"
>the regular expression would be: 
<CODE
CLASS="literal"
>^T[a-z][aeiou]</CODE
><IMG
SRC="../chars/squ.gif"
ALT=" ">.</P
><P
CLASS="para"
>[To be specific:
A range is a contiguous series of characters, from low to high, in the
<SPAN
CLASS="link"
>ASCII chart (<A
CLASS="linkend"
HREF="ch51_03.htm"
TITLE="ASCII Characters: Listing and Getting Values "
>51.3</A
>)</SPAN
>.
For example, <CODE
CLASS="literal"
>[z-a]</CODE
> is <EM
CLASS="emphasis"
>not</EM
> a range because it's backwards.
The range <CODE
CLASS="literal"
>[A-z]</CODE
> does match both uppercase and lowercase letters,
but it also matches the six characters that fall between uppercase
and lowercase letters in the ASCII chart:
<CODE
CLASS="literal"
>[</CODE
>, <CODE
CLASS="literal"
>\</CODE
>, <CODE
CLASS="literal"
>]</CODE
>, <CODE
CLASS="literal"
>^</CODE
>, <CODE
CLASS="literal"
>_</CODE
>, and <CODE
CLASS="literal"
>`</CODE
>.
<EM
CLASS="emphasis"
>-JP</EM
>&nbsp;]</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-427-SECT-1.5"
>26.4.5 Exceptions in a Character Set </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-28629"
></A
><A
CLASS="indexterm"
NAME="AUTOID-28632"
></A
>You can easily search for all characters except those in square
brackets by putting a
caret (<CODE
CLASS="literal"
>^</CODE
>)
as the first character after the 
left square bracket (<CODE
CLASS="literal"
>[</CODE
>).
To match all characters except lowercase vowels use: 
<CODE
CLASS="literal"
>[^aeiou]</CODE
>.</P
><P
CLASS="para"
>Like the anchors in places that can't be considered an anchor, the
right square bracket (<CODE
CLASS="literal"
>]</CODE
>)
and
dash (<CODE
CLASS="literal"
>-</CODE
>)
do not have a special meaning if they directly follow 
a&nbsp;<CODE
CLASS="literal"
>[</CODE
>.
<A
CLASS="xref"
HREF="ch26_04.htm#UPT-ART-427-TAB-1"
TITLE="Regular Expression Character Set Examples"
>Table 26.2</A
>
has some examples.&#13;</P
><TABLE
CLASS="table"
><CAPTION
CLASS="table"
><A
CLASS="title"
NAME="UPT-ART-427-TAB-1"
>Table 26.2: Regular Expression Character Set Examples</A
></CAPTION
><THEAD
CLASS="thead"
><TR
CLASS="row"
VALIGN="TOP"
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
>Regular Expression</TH
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
>Matches</TH
></TR
></THEAD
><TBODY
CLASS="tbody"
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[0-9]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any digit</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[^0-9]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any character other than a digit</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[-0-9]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any digit or a <CODE
CLASS="literal"
>-</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[0-9-]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any digit or a <CODE
CLASS="literal"
>-</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[^-0-9]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any character except a digit or a <CODE
CLASS="literal"
>-</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[]0-9]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any digit or a <CODE
CLASS="literal"
>]</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[0-9]]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any digit followed by a <CODE
CLASS="literal"
>]</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[0-99-z]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Any digit or any character <SPAN
CLASS="link"
>between 
9 and z (<A
CLASS="linkend"
HREF="ch51_03.htm"
TITLE="ASCII Characters: Listing and Getting Values "
>51.3</A
>)</SPAN
></P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>[]0-9-]</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any digit, a <CODE
CLASS="literal"
>-</CODE
>, or a <CODE
CLASS="literal"
>]</CODE
></TD
></TR
></TBODY
></TABLE
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-427-SECT-1.6"
>26.4.6 Repeating Character Sets with <CODE
CLASS="literal"
>*</CODE
> </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-28691"
></A
>The third part of a regular expression is the modifier.
It is used to specify how many times you expect to see the previous
character set. The special character <CODE
CLASS="literal"
>*</CODE
>&nbsp;(asterisk)
matches
<EM
CLASS="emphasis"
>zero or more</EM
>
copies.
That is, the regular expression
<CODE
CLASS="literal"
>0*</CODE
>
matches 
zero or more zeros,
while the expression
<CODE
CLASS="literal"
>[0-9]*</CODE
>
matches zero or more digits.</P
><P
CLASS="para"
>This explains why the pattern
<CODE
CLASS="literal"
>^#*</CODE
>
is useless, as it matches any number of 
<CODE
CLASS="literal"
>#</CODE
>'s
at the beginning of the line, including 
<EM
CLASS="emphasis"
>zero</EM
>.
Therefore, this will match every line, because every line starts with
zero or more 
<CODE
CLASS="literal"
>#</CODE
>'s.</P
><P
CLASS="para"
>At first glance, it might seem that starting the count at zero is
stupid.
Not so.
Looking for an unknown number of characters is very important.
Suppose you wanted to look for a digit at the beginning of a line,
and there may or may not be spaces before the digit.
Just use <CODE
CLASS="literal"
>^</CODE
><IMG
SRC="../chars/squ.gif"
ALT=" "><CODE
CLASS="literal"
>*</CODE
>
to match zero or more spaces at the beginning of the line.
If you need to match one or more, just repeat the character set.
That is, 
<CODE
CLASS="literal"
>[0-9]*</CODE
>
matches zero or more digits and
<CODE
CLASS="literal"
>[0-9][0-9]*</CODE
>
matches one or more digits.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-427-SECT-1.7"
>26.4.7 Matching a Specific Number of Sets with \&nbsp;{ and \&nbsp;} </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-28711"
></A
><A
CLASS="indexterm"
NAME="AUTOID-28714"
></A
>You cannot specify a maximum number of sets
with the
<CODE
CLASS="literal"
>*</CODE
>
modifier.
However, 
<SPAN
CLASS="link"
>some programs (<A
CLASS="linkend"
HREF="ch26_09.htm"
TITLE="Valid Metacharacters for Different UNIX Programs "
>26.9</A
>)</SPAN
>
recognize a 
special pattern you can use to specify the
minimum and maximum number of repeats. 
This is done by putting those two numbers between 
<CODE
CLASS="literal"
>\{</CODE
>
and
<CODE
CLASS="literal"
>\}</CODE
>.</P
><P
CLASS="para"
>Having convinced you that 
<CODE
CLASS="literal"
>\{</CODE
>
isn't a plot to confuse you, an example is in order. The regular
expression to match four, five, six, seven, or eight lowercase letters is:
<CODE
CLASS="literal"
>[a-z]\{4,8\}</CODE
>.
Any numbers between 0 and 255 can be used.
The second number may be omitted, which removes the upper limit.
If the comma and the second number are omitted, the pattern must be
duplicated the exact number of times specified by the first number.</P
><BLOCKQUOTE
CLASS="caution"
><P
CLASS="para"
><STRONG
>CAUTION:</STRONG
> The backslashes deserve a special discussion.
Normally a backslash 
<EM
CLASS="emphasis"
>turns off</EM
>
the special meaning for a character.
For example, a literal period is matched by 
<CODE
CLASS="literal"
>\.</CODE
>
and a literal asterisk is matched by
<CODE
CLASS="literal"
>\*</CODE
>.

However, if a backslash is placed before a
<CODE
CLASS="literal"
>&lt;</CODE
>,
<CODE
CLASS="literal"
>&gt;</CODE
>,
<CODE
CLASS="literal"
>{</CODE
>,
<CODE
CLASS="literal"
>}</CODE
>,
<CODE
CLASS="literal"
>(</CODE
>, or
<CODE
CLASS="literal"
>)</CODE
>
or before a digit, the backslash
<EM
CLASS="emphasis"
>turns on</EM
>
a special meaning.
This was done because these special functions were added late in the
life of regular expressions. 
Changing the meaning of 
<CODE
CLASS="literal"
>{</CODE
>, <CODE
CLASS="literal"
>}</CODE
>,
<CODE
CLASS="literal"
>(</CODE
>, <CODE
CLASS="literal"
>)</CODE
>,
<CODE
CLASS="literal"
>&lt;</CODE
>, and <CODE
CLASS="literal"
>&gt;</CODE
>
would have broken old expressions. (This is a horrible crime punishable
by a year of hard labor writing COBOL programs.)
Instead, adding a backslash added functionality without breaking old
programs. Rather than complain about the change, view it as evolution.&#13;</P
></BLOCKQUOTE
><P
CLASS="para"
>You must remember that modifiers like
<CODE
CLASS="literal"
>*</CODE
>
and
<CODE
CLASS="literal"
>\{1,5\}</CODE
>
only act as modifiers if they follow a character set.
If they were at the beginning of a pattern, they would not be modifiers.
<A
CLASS="xref"
HREF="ch26_04.htm#UPT-ART-427-TAB-2"
TITLE="Regular Expression Pattern Repetition Examples"
>Table 26.3</A
>
is a list of examples, and the exceptions.&#13;</P
><TABLE
CLASS="table"
><CAPTION
CLASS="table"
><A
CLASS="title"
NAME="UPT-ART-427-TAB-2"
>Table 26.3: Regular Expression Pattern Repetition Examples</A
></CAPTION
><THEAD
CLASS="thead"
><TR
CLASS="row"
VALIGN="TOP"
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
>Regular Expression</TH
><TH
CLASS="entry"
ALIGN="LEFT"
ROWSPAN="1"
COLSPAN="1"
>Matches</TH
></TR
></THEAD
><TBODY
CLASS="tbody"
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>*</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line with a <CODE
CLASS="literal"
>*</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>\*</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line with a <CODE
CLASS="literal"
>*</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>\\</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line with a <CODE
CLASS="literal"
>\</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^*</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line starting with a <CODE
CLASS="literal"
>*</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^A*</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^A\*</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line starting with an A<CODE
CLASS="literal"
>*</CODE
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^AA*</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line starting with one A</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^AA*B</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Any line starting with one or more A's followed
by a B</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^A\{4,8\}B</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Any line starting with four, five, six, seven, or eight A's 
followed by a B</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^A\{4,\}B</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
><P
CLASS="para"
>Any line starting with four or more A's followed
by a B</P
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>^A\{4\}B</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line starting with an AAAAB</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>\{4,8\}</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line with a {4,8}</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>A{4,8}</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>Any line with an A{4,8}</TD
></TR
></TBODY
></TABLE
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-427-SECT-1.8"
>26.4.8 Matching Words with \&nbsp;&lt; and \&nbsp;&gt; </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-28804"
></A