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Introduction Regular Expressions are a widely-used method of specifying patterns of text to search for. Special metacharacters allow to specify, for instance, that a particular string you are looking for occurs at the beginning or end of a line, or contains n recurrences of a certain character. Regular expressions look ugly for novices, but really they are very simple (well, usually simple ;) ), handly and powerfull tool. Let's start our learning trip! Simple matches Any single character matches itself, unless it is a metacharacter with a special meaning described below. A series of characters matches that series of characters in the target string, so the pattern "bluh" would match "bluh'' in the target string. Quite simple, eh ? You can cause characters that normally function as metacharacters or escape sequences to be interpreted literally by 'escaping' them by preceding them with a backslash "\", for instance: metacharacter "^" match beginning of string, but "\^" match character "^", "\\" match "\" and so on. Examples: Escape sequences Characters may be specified using a escape sequences syntax much like that used in C and Perl: "\n'' matches a newline, "\t'' a tab, etc. More generally, \xnn, where nn is a string of hexadecimal digits, matches the character whose ASCII value is nn. If you need wide (Unicode) character code, you can use '\x{nnnn}', where 'nnnn' - one or more hexadecimal digits.
Examples: foo\x20bar matchs 'foo bar' (note space in the middle) Character classes You can specify a character class, by enclosing a list of characters in [], which will match any one character from the list. If the first character after the "['' is "^'', the class matches any character not in the list. Examples: Within a list, the "-'' character is used to specify a range, so that a-z represents all characters between "a'' and "z'', inclusive. If you want "-'' itself to be a member of a class, put it at the start or end of the list, or escape it with a backslash. If You want ']' you may place it at the start of list or escape it with a backslash. Examples: Metacharacters Metacharacters are special characters which are the essence of Regular Expressions. There are different types of metacharacters, described below. Metacharacters - line separators
Examples: The "^" metacharacter by default is only guaranteed to match at the beginning of the input string/text, the "$" metacharacter only at the end. Embedded line separators will not be matched by "^'' or "$''. The \A and \Z are just like "^'' and "$'', except that they won't match multiple times when the modifier /m is used, while "^'' and "$'' will match at every internal line separator. The ".'' metacharacter by default matches any character, but if you switch Off the modifier /s, then '.' won't match embedded line separators. "^" is at the beginning of a input string, and, if modifier /m is On, also immediately following any occurrence of \x0D\x0A or \x0A or \x0D. Note that there is no empty line within the sequence \x0D\x0A. "$" is at the end of a input string, and, if modifier /m is On, also immediately preceding any occurrence of \x0D\x0A or \x0A or \x0D. Note that there is no empty line within the sequence \x0D\x0A. "." matchs any character, but if You switch Off modifier /s then "." doesn't match \x0D\x0A and \x0A and \x0D. Metacharacters - predefined classes
You may use \w, \d and \s within custom character classes. Examples: Metacharacters - word boundaries \b Match a word boundary A word boundary (\b) is a spot between two characters that has a \w on one side of it and a \W on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a \W. Metacharacters - iterators Any item of a regular expression may be followed by another type of metacharacters - iterators. Using this metacharacters You can specify number of occurences of previous character, metacharacter or subexpression. * zero or more ("greedy"), similar to {0,} {n} exactly n times ("greedy") So, digits in curly brackets of the form {n,m}, specify the minimum number of times to match the item n and the maximum m. The form {n} is equivalent to {n,n} and matches exactly n times. The form {n,} matches n or more times. There is no limit to the size of n or m, but large numbers will chew up more memory and slow down r.e. execution. If a curly bracket occurs in any other context, it is treated as a regular character. Examples: A little explanation about "greediness". "Greedy" takes as many as possible, "non-greedy" takes as few as possible. For example, 'b+' and 'b*' applied to string 'abbbbc' return 'bbbb', 'b+?' returns 'b', 'b*?' returns empty string, 'b{2,3}?' returns 'bb', 'b{2,3}' returns 'bbb'. You can switch all iterators into "non-greedy" mode (see the modifier /g). Metacharacters - alternatives You can specify a series of alternatives for a pattern using "|'' to separate them, so that fee|fie|foe will match any of "fee'', "fie'', or "foe'' in the target string (as would f(e|i|o)e). The first alternative includes everything from the last pattern delimiter ("('', "['', or the beginning of the pattern) up to the first "|'', and the last alternative contains everything from the last "|'' to the next pattern delimiter. For this reason, it's common practice to include alternatives in parentheses, to minimize confusion about where they start and end. Alternatives are tried from left to right, so the first alternative found for which the entire expression matches, is the one that is chosen. This means that alternatives are not necessarily greedy. For example: when matching foo|foot against "barefoot'', only the "foo'' part will match, as that is the first alternative tried, and it successfully matches the target string. (This might not seem important, but it is important when you are capturing matched text using parentheses.) Also remember that "|'' is interpreted as a literal within square brackets, so if You write [fee|fie|foe] You're really only matching [feio|]. Examples: Metacharacters - subexpressions The bracketing construct ( ... ) may also be used for define r.e. subexpressions. Subexpressions are numbered based on the left to right order of their opening parenthesis. Examples: Metacharacters - backreferences Metacharacters \1 through \9 are interpreted as backreferences. \<n> matches previously matched subexpression #<n>. Examples: Modifiers Modifiers are for changing behaviour of regular expressions parser. i Do case-insensitive pattern matching (using installed in you system locale settings), see also InvertCase. m Treat string as multiple lines. That is, change "^'' and "$'' from matching at only the very start or end of the string to the start or end of any line anywhere within the string, see also Line separators. s Treat string as single line. That is, change ".'' to match any character whatsoever, even a line separators (see also Line separators), which it normally would not match. g Non standard modifier. Switching it Off You'll switch all following operators into non-greedy mode (by default this modifier is On). So, if modifier /g is Off then '+' works as '+?', '*' as '*?' and so on Perl extensions (?imsxr-imsxr) Examples: (?#text) This text is (c) by Andrey V. Sorokin (Saint-Peterburg, Russia) |
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