Regex Tutorial

• a, X, 9, < -- ordinary characters just match themselves exactly. The meta-characters which do not match themselves because they have special meanings are: . ^ $ * + ? { [ ] \ | ( ) (details below)
• . (a period) -- matches any single character except newline '\n'
• \w -- (lowercase w) matches a "word" character: a letter or digit or underbar [a-zA-Z0-9_]. Note that although "word" is the mnemonic for this, it only matches a single word char, not a whole word. \W (upper case W) matches any non-word character.
• \b -- boundary between word and non-word
• \s -- (lowercase s) matches a single whitespace character -- space, newline, return, tab, form [ \n\r\t\f]. \S (upper case S) matches any non-whitespace character.
• \t, \n, \r -- tab, newline, return
• \d -- decimal digit [0-9] (some older regex utilities do not support but \d, but they all support \w and \s)
• ^ = start, $ = end -- match the start or end of the string
• ** \ ** -- inhibit the "specialness" of a character. So, for example, use . to match a period or \ to match a slash. If you are unsure if a character has special meaning, such as '@', you can put a slash in front of it, \@, to make sure it is treated just as a character.

Escape sequences

There are some special characters starting with backslash \ which are known as the escape or control sequences. They do not have corresponding symbols and cannot be found on a keyboard. To represent such characters we use a pair of regular symbols. In a program, this pair will be considered as exactly one character with the appropriate code.

• '\n' is the newline character;
• '\t' is the tab character;
• '\r' is the carriage return character;
• '\' is the backslash character itself;
• '\'' is the single quote mark;
• '"' is the double quote mark.