Regular Expression


Meta Characters


“ [ ] “

Specifies a Character class.

[aefg] à Matches any of the character a, e, f, g.

[a-f] à Matches any of the character in the range a to f.

[mkl$] à Matches any of the character including $. Meta characters inside character class is stripped off its special nature.

[^a-z] à Complementing; Matches all the characters other than Lower case alphabets.

“ \ “

Gives special meaning to various characters. Also used to escape all the meta characters from their special nature.

\d à matches any decimal digit; this is equivalent to the class [0-9].
\D à matches any non-digit character; this is equivalent to the class [ˆ0-9].
\s à Matches any whitespace character; this is equivalent to the class [ \t\n\r\f\v].
\S à matches any non-whitespace character; this is equivalent to the class [ˆ\t\n\r\f\v].

\w à matches any alphanumeric character; this is equivalent to the class [a-zA-Z0-9].

\W à matches any non-alphanumeric character; this is equivalent to the class [ˆa-zA-Z0-9].[\s,!] à Backslash characters can be used inside a character class. This is a character
class that match any white space character followed by “,” and “!”.

\b à matches start of end of word boundary.

\B à opposite of \b.

\A à matches start of the string

\Z à matches end of the string

“ . “

This matches anything except new line.

“ * “

Pre-pended character can be matched zero or more number of times.

Ca*t à Matches Ct, Cat, Caat, Caaat, Caaaat and so on.

Note :
Repetitions such as d*c are greedy; when repeating a RE, the matching engine
will try to repeat it as many times as possible. If later portions of the pattern don’t
match, the matching engine will then back up and try again with few repetitions.

“ + “

Pre-pended character can be matched once or more number of times.

Ca*t à Matches Cat, Caat, Caaat, Caaaat and so on but not Ct.

“ ? “

Pre-pended character can be matched zero or once.

Ca*t à Matches only Ct or Cat.

“ {m,n} “

Pre-pended character can be matched minimum of m times but maximum of n
times. Either m or n can be omitted. m is assumed to be 0 if omitted. n is assumed to be infinity if omitted.

Ca{1,3}t à Matches Cat, Caat, Caaat but not Ct, Caaaat, Caaaat and so on.

Note : {0,} is same as *. {1,} is same as +. {,1} is same as ?

“ “

The or operator which can be used between two regular expressions.

Chandra Shekar à matches either Chandra or Shekar.

“ ^ “

Matches beginning of the line or beginning of a string depending upon mode.

“ $ “

Matches end of the line or end of a string depending upon mode.

“ ( ) “

Used to group characters so that repetition meta-characters can be used over group.

Ch(an)*dra à matches Chdra, Chandra, Chanandra, Chananandra… so on.



Comments

Post a Comment

Popular posts from this blog

AWK - quick reference

AWK awk is an extremely versatile pattern matching and processing language. Although "awk scripts" can be written simply on the command line, more sophisticated use involves script files. Where the shell language is geared to running various utilities, the awk language is geared to extracting information from files according to rules and patterns. As a programming language (which looks much like C in some respects), awk has the usual selection, loop, and condition constructs, and has variables. Awk variables are not referenced with a leading $ as in the shell, except for the variables representing fields in the input line: $1, $2, ... . See below. As with the shell, comments come after the # character. To invoke an awk script is similar to the first method for shell scripts (an awk script cannot be made executable, however, an executable shell script with a here-document for awk could be used): awk -f script-file input-files If is missing, awk looks for input from stdin. Th
Read more

GDB - A quick reference

GDB - A quick reference gdb program-name gdb program-name core-file gdb program-name process-id - GDB gets attached to the running process with its id equal to given process-id; Note that the running process pauses. Now some variable value can be set and process can be detached. (gdb prompt) attach process-id - Attaches GDB to given process ID (gdb prompt) detach - detaches the process attached. gdb -q -q stands for quite; No copyright printed b(break) main - Put a breakpoint at the beginning of the program b(break) - Put a breakpoint at the next line ( current line makes no sense) b(break) N - Put a breakpoint at line N b(break) filename:N - Put a breakpoint at line N in file filename b(break) class::func - To put a break point in C++ function b(break) +N - Put a breakpoint N lines down from the current line b(break) -N - Put a breakpoint N lines up from the current line b(break) fn - Put a breakpoint at the beginning of function "fn" b(break) if (exprsn) - Any for
Read more

Linux Kernel Hacking!!

Linux Kernel Hacking!! In this first of a two-part series, learn about system and environment requirements, the best ways to acquire Linux™ source code, how to configure and boot your new kernel, and how to use the printk function to print messages during bootup. Before you start Learn what these tutorials can teach you, and what you need to run the examples in them. About this series The capability of being modified is perhaps one of Linux's greatest strengths, and anyone who has dabbled with the source code has at least stood at the gates of the kingdom, if not opened them up and walked inside. These two tutorials are intended to get you started. They are for anyone who knows a little bit of programming and who wants to contribute to the development of Linux, who feels that something is missing in the kernel and wants to fix that, or who just wants to find out how a real operating system works. About this tutorial This tutorial takes a basic approach to
Read more