Jump to content
Muxe Inc Forums
Sign in to follow this  


Recommended Posts

Garl    0



Syntax of Regular Expressions


(Description bason on description TRegExpr(Andrey V. Sorokin).

But only description. Not code)




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



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.



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 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



\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



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.



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.



[-az] matchs 'a', 'z' and '-'

[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.

[\d-t] matchs any digit, '-' or 't'.

[]-a] matchs any char from ']'..'a'.





Metacharacters are special characters which are the essence of Regular

Expressions. There are different types of metacharacters, described




Metacharacters - line separators


^ start of line

$ end of line

\A start of text

\Z end of text

. any character in line



^foobar matchs string 'foobar' only if it's at the beginning of line

foobar$ matchs string 'foobar' only if it's at the end of line

^foobar$ matchs string 'foobar' only if it's the only string in line

foob.r matchs strings like 'foobar', 'foobbr', 'foob1r' and so on


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 "$''.

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 ".'' metacharacter by default matches any character, but if You

switch Off the modifier /s, then '.' won't match embedded line




"^" 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 (if

You are using Unicode, then also \x2028 or \x2029 or

\x0B or \x0C or \x85). 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 (if You

are using Unicode, then also \x2028 or \x2029 or \x0B

or \x0C or \x85). Note that there is no empty line within the sequence



"." matchs any character, but if You switch Off modifier /s then "."

doesn't match \x0D\x0A and \x0A and \x0D (if You are using Unicode,

then also \x2028 and \x2029 and \x0B and \x0C and \x85).


Note that "^.*$" (an empty line pattern) doesnot match the empty string

within the sequence \x0D\x0A, but matchs the empty string within the

sequence \x0A\x0D.



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.



foob\dr matchs strings like 'foob1r', ''foob6r' and so on but not

'foobar', 'foobbr' and so on

foob[\w\s]r matchs 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 occurences of previous character, metacharacter or



* 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.



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'


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




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




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



Metacharacters - subexpressions


The bracketing construct ( ... ) may also be used for define r.e.


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

opening parenthesis.

First subexpression has number '1' (whole r.e. match '$&').



(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.



Metacharacters - backreferences


Metacharacters \1 through \9 are interpreted as backreferences. \

matches previously matched subexpression #.



(.)\1+ matchs 'aaaa' and 'cc'.

(.+)\1+ also match 'abab' and '123123'

(['"]?)(\d+)\1 matchs '"13" (in double quotes), or '4' (in single

quotes) or 77 (without quotes) etc





There are many ways to set up modifiers.

Any of these modifiers may be embedded within the regular expression

itself using the (?...) construct.




Do case-insensitive pattern matching (using installed in you system

locale settings), see also InvertCase.





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.






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.





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





Extend your pattern's legibility by permitting whitespace and

comments (see explanation below).


The modifier /x itself needs a little more explanation. It tells to

ignore whitespace that is neither backslashed nor within a

character class. You can use this to break up your regular expression

into (slightly) more readable parts. The # character is also treated as a

metacharacter introducing a comment, for example:




(abc) # comment 1


| # You can use spaces to format r.e. - ignores it


(efg) # comment 2





This also means that if you want real whitespace or # characters in the

pattern (outside a character class, where they are unaffected by /x),

that you'll either have to escape them or encode them using octal or hex

escapes. Taken together, these features go a long way towards making

regular expressions text more readable.


Perl extensions



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




(?i)Saint-Petersburg matchs 'Saint-petersburg' and 'Saint-


(?i)Saint-(?-i)Petersburg matchs 'Saint-Petersburg' but not 'Saint-


(?i)(Saint-)?Petersburg matchs 'Saint-petersburg' and 'saint-


((?i)Saint-)?Petersburg matchs 'saint-Petersburg', but not 'saint-





A comment, the text is ignored.

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this