Regular Expressions

Introduction

Regular Expressions are a widely-used method of specifying patterns of text to search for. Special metacharacters 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.

Regular expressions look ugly for novices, but really they are very simple, handy and powerful tool.

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.
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:
foobar match's string 'foobar'
\^FooBarPtr match's '^FooBarPtr'

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.

\xnn char with hex code nn
\x{nnnn} char with hex code nnnn (one byte for plain text and two bytes for Unicode)
\t tab (HT/TAB), same as \x09
\n newline (NL), same as \x0a
\r car.return (CR), same as \x0d
\f form feed (FF), same as \x0c
\a alarm (bell) (BEL), same as \x07
\e escape (ESC), same as \x1b

Examples:
foo\x20bar match's 'foo bar' (note space in the middle)
\tfoobar matchs 'foobar' predefined by tab

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:
foob[aeiou]r finds strings 'foobar', 'foober' etc. but not 'foobbr', 'foobcr' etc.
foob[^aeiou]r find strings 'foobbr', 'foobcr' etc. but not 'foobar', 'foober' etc.

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:
[-az] match's 'a', 'z' and '-'
[az-] match's 'a', 'z' and '-'
[a\-z] match's 'a', 'z' and '-'
[a-z] match's all twenty six small characters from 'a' to 'z'
[\n-\x0D] match's any of #10,#11,#12,#13.
[\d-t] match's any digit, '-' or 't'.
[]-a] match's any char from ']'..'a'.

Metacharacters

Metacharacters are special characters which are the essence of Regular Expressions. There are different types of metacharacters, described below.

Metacharacters - line separators

^ start of line
$ end of line
\A start of text
\Z end of text
. any character in line

Examples:
^foobar match's string 'foobar' only if it's at the beginning of line
foobar$ match's string 'foobar' only if it's at the end of line
^foobar$ match's string 'foobar' only if it's the only string in line
foob.r match's strings like 'foobar', 'foobbr', 'foob1r' and so on

Metacharacters - predefined classes

\w an alphanumeric character (including "_")
\W a nonalphanumeric
\d a numeric character
\D a non-numeric
\s any space (same as [ \t\n\r\f])
\S a non space

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

Examples:
foob\dr match's strings like 'foob1r', ''foob6r' and so on but not 'foobar', 'foobbr' and so on
foob[\w\s]r match's strings like 'foobar', 'foob r', 'foobbr' and so on but not 'foob1r', 'foob=r' and so on

Metacharacters - word boundaries

\b Match a word boundary
\B Match a non-(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 occurrences of previous character, metacharacter or sub expression.

* zero or more ("greedy"), similar to {0,}
+ one or more ("greedy"), similar to {1,}
? zero or one ("greedy"), similar to {0,1}
{n} exactly n times ("greedy")
{n,} at least n times ("greedy")
{n,m} at least n but not more than m times ("greedy")
*? zero or more ("non-greedy"), similar to {0,}?
+? one or more ("non-greedy"), similar to {1,}?
?? zero or one ("non-greedy"), similar to {0,1}?
{n}? exactly n times ("non-greedy")
{n,}? at least n times ("non-greedy")
{n,m}? at least n but not more than m times ("non-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:
foob.*r match's strings like 'foobar', 'foobalkjdflkj9r' and 'foobr'
foob.+r match's strings like 'foobar', 'foobalkjdflkj9r' but not 'foobr'
foob.?r match's strings like 'foobar', 'foobbr' and 'foobr' but not 'foobalkj9r'
fooba{2}r match's the string 'foobaar'
fooba{2,}r matchs strings like 'foobaar', 'foobaaar', 'foobaaaar' etc.
fooba{2,3}r match's strings like 'foobaar', or 'foobaaar' but not 'foobaaaar'

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 using the (?g) switch

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:
foo(bar|foo) match's strings 'foobar' or 'foofoo'.

Metacharacters - sub expressions

The bracketing construct ( ... ) may also be used for define r.e. sub expressions.
Sub expressions are numbered based on the left to right order of their opening parenthesis.
First sub expression has number '1'

Examples:
(foobar){8,10} match's strings which contain 8, 9 or 10 instances of the 'foobar'
foob([0-9]|a+)r match's 'foob0r', 'foob1r' , 'foobar', 'foobaar', 'foobaar' etc.

Metacharacters - backreferences

Metacharacters \1 through \9 are interpreted as backreferences. \<n> matches previously matched sub expression #<n>.

Examples:
(.)\1+ match's 'aaaa' and 'cc'.
(.+)\1+ also match 'abab' and '123123'
(['"]?)(\d+)\1 match's '"13" (in double quotes), or '4' (in single quotes) or 77 (without quotes) etc

Perl extensions

You may use it into r.e. for modifying modifiers by the fly. If this construction inlined into sub expression, then it effects only into this sub expression

Examples:
(?i)Saint-Petersburg match's 'Saint-petersburg' and 'Saint-Petersburg'
(?i)Saint-(?-i)Petersburg match's 'Saint-Petersburg' but not 'Saint-petersburg'
(?i)(Saint-)?Petersburg match's 'Saint-petersburg' and 'saint-petersburg'
((?i)Saint-)?Petersburg match's 'saint-Petersburg', but not 'saint-petersburg'

Common Examples

Find An HTML Color Hex Value:
(?G)\#[a-fA-F0-9]{3,6} Matchs #FFA and #FF00FF

Find a Percentage:
[\+\-]?[\.]?[0-9]+[\.]?[0-9]*\% Matchs 12% and 100%

Find an e-mail:
(?G)[\w\d\-\.]+@[\w\d\-]+(\.[\w\d\-]+)+ Matchs testing@test.com