Syntax of Regular Expressions



Introduction


Regular Expressions are 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 a number of  recurrences of a certain character.


Regular expressions look ugly for the novice user, but really they are very simple, handy and powerful.



Simple matches


Any single character matches itself, unless it's a meta-character with a special meaning as 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 meta-characters or escape sequences to be interpreted literally by 'escaping' them by preceding them with a backslash "\", for instance: meta-character "^" matches the beginning of a string, but "\^" matches characters "^", "\" and so on.


Examples

texnotes                matches the string: "texnotes"
\^texnotes                matches "^texnotes"



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                matches 'foo bar' (note the space in the middle).
\tfoobar                matches '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]             matches 'a', 'z' and '-'
[az-]             matches 'a', 'z' and '-'
[a\-z]            matches 'a', 'z' and '-'
[a-z]             matches all twenty six small characters from 'a' to 'z'
[\n-\x0D]         matches any of #10,#11,#12,#13.
[\d-t]            matches any digit, '-' or 't'.
[]-a]             matches any char from ']'..'a'.



Meta-characters


Meta-characters are special characters which are the essence of Regular Expressions. There are different types of meta-characters, described below.


Meta-characters - line separators

^      start of line

$      end of line

\A    start of text

\Z    end of text

.      any character in line


Examples

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


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

You may, however, wish to treat a string as a multi-line buffer, such that the "^'' will match after any line separator within the string, and "$'' will match before any line separator. You can do this by switching On the modifier /m.


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.


The Regular Expression engine works with line separators as recommended at:

www.unicode.org ( http://www.unicode.org/unicode/reports/tr18/ )


"^" 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


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


The Regular Expression engine uses properties SpaceChars and WordChars to define character classes \w, \W, \s, \S, so you can easily redefine it.



Meta-characters - 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.



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 number of occurrences of previous character, meta-character 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                    matches strings like 'foobar', 'foobalkjdflkj9r' and 'foobr'
foob.+r                    matches strings like 'foobar', 'foobalkjdflkj9r' but not 'foobr'
foob.?r                    matches strings like 'foobar', 'foobbr' and 'foobr' but not 'foobalkj9r'
fooba{2}r                  matches the string 'foobaar'
fooba{2,}r                matches strings like 'foobaar', 'foobaaar', 'foobaaaar' etc.
fooba{2,3}r                matches 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 (see the modifier /g).



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

foo(bar|foo)        matches strings 'foobar' or 'foofoo'.



Meta-characters - sub-expressions


Subexpressions are numbered based on the left to right order of their opening parenthesis.


Examples

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



Meta-characters - backreferences


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


Examples

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



Modifiers

       

Modifiers are for changing the behavior of the Regular Expression.


There are many ways to set up modifiers.

Any of these modifiers may be embedded within the regular expression itself using the (?...) construct.


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 


x

Extend your pattern's legibility by permitting whitespace and comments (see explanation below). 


r

Non-standard modifier. If is set then range - additional include Russian letter '', - additional include '', and - include all Russian symbols.



Perl extensions


(?imsxr-imsxr)

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


Examples
(?i)Saint-Petersburg        matches 'Saint-petersburg' and 'Saint-Petersburg'
(?i)Saint-(?-i)Petersburg        matches 'Saint-Petersburg' but not 'Saint-petersburg'
(?i)(Saint-)?Petersburg        matches 'Saint-petersburg' and 'saint-petersburg'
((?i)Saint-)?Petersburg        matches 'saint-Petersburg', but not 'saint-petersburg'
(?#text)                        A comment, the text is ignored.
                               Note that Regular Expression engine closes the comment as soon as it sees a ")",
                               so there is no way to put a literal ")" in the comment.