%ndn%\doc\REGEXPS.TXT
Syntax of Regular Expressions
(Description bason on description TRegExpr(Andrey V. Sorokin).
But only description. Not code)
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 (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.
Examples:
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
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 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.
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] 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
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 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 "$'' will
match at every internal line separator.
The ".'' metacharacter by default matches any character, but if You
switch Off the modifier /s, then '.' won't match embedded line
separators.
"^" is at the beginning of a input string, and, if modifier /m is On,
also immediately following any occurrence of \x0D\x0A or \x0A or \x0D (if
You are using Unicode, then also \x2028 or \x2029 or
\x0B or \x0C or \x85). Note that there is no empty line within the
sequence \x0D\x0A.
"$" 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
\x0D\x0A.
"." 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.
Examples:
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
subexpression.
* 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 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
/g).
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) matchs strings 'foobar' or 'foofoo'.
Metacharacters - subexpressions
The bracketing construct ( ... ) may also be used for 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 '$&').
Examples:
(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 #.
Examples:
(.)\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
Modifiers
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).
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
(?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 matchs 'Saint-petersburg' and 'Saint-
Petersburg'
(?i)Saint-(?-i)Petersburg matchs 'Saint-Petersburg' but not 'Saint-
petersburg'
(?i)(Saint-)?Petersburg matchs 'Saint-petersburg' and 'saint-
petersburg'
((?i)Saint-)?Petersburg matchs 'saint-Petersburg', but not 'saint-
petersburg'
(?#text)
A comment, the text is ignored.
