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Introduction to Perl for Unix System Administrators

(Perl without excessive complexity)

by Dr Nikolai Bezroukov

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5.8. Summary and Reference

This chapter has been a long one primarily because 1) regular expressions are powerful, 2) regular expressions are heavily integrated into the rest of the language, and 3) people get confused about regular expressions and seldom use them correctly. If you learn their secrets, they make your life a lot easier, especially if you deal with large amounts of data.

 

Meta Brackets	Description
()	The parentheses let you affect the order of pattern evaluation and act as a form of pattern memory. See the section "Pattern Memory" later in this chapter for more information.
(?...)	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.

 

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

Modifiers

Learn well the principles in this chapter. In order to use regular expressions effectively, you need first to learn to construct simple expressions or you will be spinning your wheels quite a lot. Then try to modify more complex from books and scripts on the Net and only then begin to construct complex regular expressions yourself. . You really need to work from simple to complicated. Let your knowledge about regular expressions grow naturally. Start with simple regular expressions, and as your knowledge grows, let the complexity of your regular expressions grow.

Be sure to check out the documentation in perlref. This chapter is pretty superficial and contains material that just will get you started and give you some skill in regular expression manipulation. there is much more material here that we can cover in the introduction.

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