# Regular Expressions

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 novice users, but they are very powerful tools.

Simple matches:

Any single character matches itself, unless it is a metacharacter with a special meaning described below. 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.

foobar | matchs string "foobar"
\^FooBarPtr | matchs "^FooBarPtr"

Escape sequences:

Characters may be specified using a escape sequences syntax much like that used in 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. 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} | two bytes char with hex code nnnn (unicode)
\t | tab (HT/TAB), same as \x09
\n | new line (NL), same as \x0a
\r | carriage 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
foo\x20bar | matchs "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. 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.

[-az] | matchs "a", "z" and "-"
[a\-z] | matchs "a", "z" and "-"
[a-z] | matchs all twenty six small characters from "a" to "z"
[\n-\x0D] | matchs any of #10,#11,#12,#13
[^0-9] | matchs any none digit character
[\d-t] | matchs any digit, '-' or 't'
[]-a] | matchs any char from ']'..'a'
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.

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
. | any character in line

Word/Text 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.

\b | word boundary
\B | non-(word boundary)
\A | start of text
\Z | end of text

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. 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. 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'.

* | 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")
foob.*r | matchs strings like 'foobar', 'foobalkjdflkj9r' and 'foobr'
foob.+r | matchs strings like 'foobar', 'foobalkjdflkj9r' but not 'foobr'
foob.?r | matchs strings like 'foobar', 'foobbr' and 'foobr' but not 'foobalkj9r'
fooba{2}r | matchs the string 'foobaar'
fooba{2,}r | matchs strings like 'foobaar', 'foobaaar', 'foobaaaar' etc.
fooba{2,3}r | matchs strings like 'foobaar', or 'foobaaar' but not 'foobaaaar'

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|].

foo(bar|foo) | matchs strings 'foobar' or 'foofoo'

Subexpressions, Backreferences, Substitution

The bracketing construct (...) may also be used to define r.e. subexpressions. Subexpressions are numbered based on the left to right order of their opening parenthesis. First subexpression has number '1' (whole r.e. match has number '0'). Metacharacters \1 through \9 are interpreted as backreferences, and match previously matched subexpression within the expression. Matched subexpressions can also be accessed in the replace operation using metacharacters $1 through $9.

(foobar){8,10} | matchs strings which contain 8, 9 or 10 instances of the 'foobar'
foob([0-9]|a+)r | matchs 'foob0r', 'foob1r' , 'foobar', 'foobaar', 'foobaar' etc.
(.)\1+ | matchs 'aaaa' and 'cc'
(.+)\1+ | matchs 'aaaa', 'cc', 'abab', '123123'

Substitution Example:

- Expression: (.*) (.*) Replace: $2, $1 - will switch two words around and will put a comma after the resulting first word, i.e.: if input string is "John Smith", then output will be "Smith, John".

More Information:

www.regexpstudio.com
www.regular-expressions.info