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Introduction to Perl 5.10 for Unix System Administrators

(Perl 5.10 without excessive complexity)

by Dr Nikolai Bezroukov

Contents : Foreword : Ch01 : Ch02 : Ch03 : Ch04 : Ch05 : Ch06 : Ch07 : Ch08 :

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5.8. Summary

Two general observation:

Regular expressions are powerful language that is completely distinct from "rest of Perl". It requires separate study and complex regex require separate testing.
There are a lot of fine points that you need to understand to use regular expressions correctly. Try to limit yourself to simple regex. Leave complex regex to the authors of books about Perl. That provides a nice material for their errata ;-).

To be successful you need to use to adopt some elements of style that makes mistakes less probable:

Use non-gready(lazy) matching when you do not need greedy matching. That prevent a lot of errors from happening in case you have several instances of the same pattern in the strong and do not want to find the last one but the first match.
If possible try to anchor you regex to either start of the string or to the end of the string or both. That makes matching faster and simplify debugging.
Always debug complex regex in a separate test program with multiple samples, preferably extracted from your data. Never assume that complex regex is correct on first iteration
Use extended syntax to commend and pretty print complex regex.
Try to split complex regex into two or more to make it more simple. Don't try to create a "killer" oneliner everybody will admire and nobody understand.
If you use variables $1, $1 and so on remember that in case string does not match they preserve their old values. Always check if regex is successful if you use $1. $2 and so on.
Form ($a, $b, $c)=($string=~/(\d+)\.(\d+)\.(\d+)/ is preferable to usage of separate assignment of each group via $1, $2 and $3.

Below are some reference tables from Medinets book:

***Regular Expression Meta-Characters, Meta-Brackets, and Meta-Sequences***
Meta Character	Description
^	This meta-character - the caret - will match the beginning of a string or if the `/m` option is used, matches the beginning of a line. It is one of two pattern anchors - the other anchor is the `$`.
.	This meta-character will match any character except for the newline unless the `/s` option is specified. If the `/s` option is specified, then the newline will also be matched.
$	This meta-character will match the end of a string or if the `/m` option is used, matches the end of a line. It is one of two pattern anchors - the other anchor is the `^`.
\|	This meta-character - called alternation - lets you specify two values that can cause the match to succeed. For instance, `m/a\|b/` means that the `$_` variable must contain the `"a"` or `"b"` character for the match to succeed.
*	This meta-character indicates that the "thing" immediately to the left should be matched 0 or more times in order to be evaluated as true.
+	This meta-character indicates that the "thing" immediately to the left should be matched 1 or more times in order to be evaluated as true.
?	This meta-character indicates that the "thing" immediately to the left should be matched 0 or 1 times in order to be evaluated as true. When used in conjunction with the `+`, `_`, `?`, or {`n`, `m`} meta- characters and brackets, it means that the regular expression should be non-greedy and match the smallest possible string.

Meta
Brackets Description

() The parentheses let you affect the order of pattern evaluation and act as a form of pattern memory.

(?...) If a question mark immediately follows the left parentheses, it indicates that an extended mode component is being specified.

{n, m} The curly braces let specify how many times the "thing" immediately to the left should be matched. {n} means that it should be matched exactly n times. {n,} means it must be matched at least n times. {n, m} means that it must be matched at least n times and not more than m times.

[] The square brackets let you create a character class. For instance, m/[abc]/ will evaluate to true if any of "a", "b", or "c" is contained in $_. The square brackets are a more readable alternative to the alternation meta-character.

Meta Brackets	Description
()	The parentheses let you affect the order of pattern evaluation and act as a form of pattern memory.
(?...)	If a question mark immediately follows the left parentheses, it indicates that an extended mode component is being specified.
{n, m}	The curly braces let specify how many times the "thing" immediately to the left should be matched. `{n}` means that it should be matched exactly n times. `{n,}` means it must be matched at least n times. `{n, m}` means that it must be matched at least n times and not more than m times.
[]	The square brackets let you create a character class. For instance, `m/[abc]/` will evaluate to true if any of `"a"`, `"b"`, or `"c"` is contained in `$_`. The square brackets are a more readable alternative to the alternation meta-character.

Meta
Sequences Description

\ This meta-character "escapes" the following character. This means that any special meaning normally attached to that character is ignored. For instance, if you need to include a dollar sign in a pattern, you must use \$ to avoid Perl's variable interpolation. Use \\ to specify the backslash character in your pattern.

\0nnn Any Octal byte.

\A This meta-sequence represents the beginning of the string. Its meaning is not affected by the /m option.

\b This meta-sequence represents the backspace character inside a character class; otherwise, it represents a word boundary. A word boundary is the spot between word (\w) and non-word(\W) characters. Perl thinks that the \W meta-sequence matches the imaginary characters off the ends of the string.

\B Match a non-word boundary.

\cn Any control character.

\d Match a single digit character.

\D Match a single non-digit character.

\e Escape.

\E Terminate the \L or \U sequence.

\f Form Feed.

\G Match only where the previous m//g left off.

\l Change the next character to lowercase.

\L Change the following characters to lowercase until a \E sequence is encountered.

\n Newline.

\Q Quote Regular Expression meta-characters literally until the \E sequence is encountered.

\r Carriage Return.

\s Match a single whitespace character.

\S Match a single non-whitespace character.

\t Tab.

\u Change the next character to uppercase.

\U Change the following characters to uppercase until a \E sequence is encountered.

\v Vertical Tab.

\w Match a single word character. Word characters are the alphanumeric and underscore characters.

\W Match a single non-word character.

\xnn Any Hexadecimal byte.

\Z This meta-sequence represents the end of the string. Its meaning is not affected by the /m option.

Meta Sequences	Description
\	This meta-character "escapes" the following character. This means that any special meaning normally attached to that character is ignored. For instance, if you need to include a dollar sign in a pattern, you must use `\$` to avoid Perl's variable interpolation. Use `\\` to specify the backslash character in your pattern.
\0nnn	Any Octal byte.
\A	This meta-sequence represents the beginning of the string. Its meaning is not affected by the `/m` option.
\b	This meta-sequence represents the backspace character inside a character class; otherwise, it represents a word boundary. A word boundary is the spot between word (`\w`) and non-word(`\W`) characters. Perl thinks that the `\W` meta-sequence matches the imaginary characters off the ends of the string.
\B	Match a non-word boundary.
\cn	Any control character.
\d	Match a single digit character.
\D	Match a single non-digit character.
\e	Escape.
\E	Terminate the \L or \U sequence.
\f	Form Feed.
\G	Match only where the previous `m//g` left off.
\l	Change the next character to lowercase.
\L	Change the following characters to lowercase until a `\E` sequence is encountered.
\n	Newline.
\Q	Quote Regular Expression meta-characters literally until the `\E` sequence is encountered.
\r	Carriage Return.
\s	Match a single whitespace character.
\S	Match a single non-whitespace character.
\t	Tab.
\u	Change the next character to uppercase.
\U	Change the following characters to uppercase until a `\E` sequence is encountered.
\v	Vertical Tab.
\w	Match a single word character. Word characters are the alphanumeric and underscore characters.
\W	Match a single non-word character.
\xnn	Any Hexadecimal byte.
\Z	This meta-sequence represents the end of the string. Its meaning is not affected by the /m option.

Regular Expressions form almost a 'language within a language' in Perl. As you can see above, they can be fairly involved, and (lets face it) if you are not familiar with them now, you are not going to learn them without practice. Therefore, we suggest the following path for learning regular expressions.

More about Modifiers

The matching operator has several options. The most useful option is probably the capability to ignore case (option i) and to iterate throuth all matches in a string (option g).

**Options for the Matching Operator**
Option	Description
g	This option finds all occurrences of the pattern in the string. You can iterate over the matches using a loop statement or put result into array
i	This option ignores the case of characters in the string.
m	This option treats the string as multiple lines. Perl does some optimization by assuming that `$_` contains a single line of input. If you know that it contains multiple newline characters, use this option to turn off the optimization.
o	This option compiles the pattern only once. You can achieve some small performance gains with this option. It should be used with variable interpolation only when the value of the variable will not change during the lifetime of the program.
s	This option treats the string as a single line.
x	This option lets you use extended regular expressions. Basically, this means that Perl will ignore whitespace that's not escaped with a backslash or within a character class. I highly recommend this option so you can use spaces to make your regular expressions more readable. See the section "Example: Extension Syntax" later in this chapter for more information.

Modifiers

The matching operator has several options. The most useful option is probably the capability to ignore case (option i) and to iterate throuth all matches in a string (option g).

**Options for the Matching Operator**
Option	Description
g	This option finds all occurrences of the pattern in the string. You can iterate over the matches using a loop statement or put result into array
i	This option ignores the case of characters in the string.
m	This option treats the string as multiple lines. Perl does some optimization by assuming that `$_` contains a single line of input. If you know that it contains multiple newline characters, use this option to turn off the optimization.
o	This option compiles the pattern only once. You can achieve some small performance gains with this option. It should be used with variable interpolation only when the value of the variable will not change during the lifetime of the program.
s	This option treats the string as a single line.
x	This option lets you use extended regular expressions. Basically, this means that Perl will ignore whitespace that's not escaped with a backslash or within a character class. I highly recommend this option so you can use spaces to make your regular expressions more readable. See the section "Example: Extension Syntax" later in this chapter for more information.

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