Regular Expressions is a widely used method of specifying patterns of text to search for. Special meta characters allow you 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.

 

Simple matches

 

Any single character matches itself, unless it is a meta character with a special meaning.

 

A series of characters matches that series of characters in the target string, so the pattern "duck" would match "duck'' in the target string.

 

You can cause characters that normally function as meta characters or escape sequences to be interpreted literally by 'escaping' them by preceding them with a backslash "\", for instance: meta character "^" 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 programming and scripting languages C and Perl: "\n'' matches a new line, "\t'' a tab, etc. More generally, \xnn, where nn is a string of hexadecimal digits, matches the character whose ASCII value is nn.

 

 

Examples:

 

 

 

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 ']' in the list you may place it at the start of list or escape it with a backslash.

 

Examples:

 

 

 

Meta characters

 

Meta characters are special characters which are the essence of Regular Expressions. There are different types of meta characters:

 

 

Meta characters - line separators

 

 

Examples:

 

 

The "^" meta character by default is only guaranteed to match at the beginning of the input string/text, the "$" meta character only at the end. Embedded line separators will not be matched by "^'' or "$''.

 

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 ".'' meta character 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.

 

"." matches any character, but if You switch Off modifier /s then "." doesn't match \x0D\x0A and \x0A and \x0D.

 

Note that "^.*$" (an empty line pattern) does not match the empty string within the sequence \x0D\x0A, but matches the empty string within the sequence \x0A\x0D.

 

 

Meta characters - predefined classes

 

 

You may use \w, \d and \s within custom character classes.

 

Examples:

 

 

 

Meta characters - iterators

 

Any item of a regular expression may be followed by another type of meta characters - iterators. Using this meta characters you can specify the number of occurrences of previous character, meta character or sub expression.

 

 

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.

 

If a curly bracket occurs in any other context, it is treated as a regular character.

 

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'.

 

Examples:

 

 

 

Meta characters - 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:

 

 

 

Meta characters - sub expressions

 

The bracketing construct ( ... ) may also be used for define sub-expressions. Sub expressions are numbered based on the left to right order of their opening parenthesis.

 

Examples:

 

 

 

Meta characters - back references

 

Meta characters \1 through \9 are interpreted as back references. \<n> matches previously matched sub expression #<n>.

 

Examples:

 

 

 

Meta characters - word boundaries

 

 

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.

 

 

Modifiers

 

Modifiers are for changing the behavior of the regular expression evaluation engine. Any of these modifiers may be embedded within the regular expression itself using the (?...) construct.

 

 

Examples:

 

 

 

 

Most content on this page is Copyright (C) Andrey V. Sorokin

 

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