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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 matches string 'foobar'
\^FooBarPtr matches '^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 matches 'foo bar' (note 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'.
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 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 "^" 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 "$'' will
match at every internal line separator.
The ".'' metacharacter by default matches any character, but if You switch >Off the modifier /s, then '.' won't match embedded line separators.
"^" is at the beginning of a input string, and, if modifier /m is On, also >immediately following any occurrence of \x0D\x0A or \x0A or \x0D and the >Unicode line separators \x2028 or \x2029 or \x0B or \x0C or \x85. Note that >there is no empty line within the sequence \x0D\x0A.
"$" 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 and the >Unicode line separators \x2028 or \x2029 or \x0B or \x0C or \x85. 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 and the Unicode line separators >\x2028 and \x2029 and \x0B and \x0C and \x85.
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.
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 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
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 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 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). >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) matches 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. The first subexpression has number '1'.
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.
Metacharacters - 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) etc
Modifiers
Modifiers are for changing behaviour of a regular expression search.
Any of these modifiers may be embedded within the regular expression itself >using the (?...) construct, e.g to turn off case-insensitive pattern
matching use (?-i). If you want to turn it on again later in the expression >use (?i).
The default modifier states in 40tude Dialog are "imsg" ("x" is not set).
i
Do case-insensitive pattern matching (using installed in you system locale >settings).
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 the
regular expression engine 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. - the r.e. engine 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 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 the regular expression engine >closes the comment as soon as it sees a ")", so there is no way to put a >literal ")" in the comment.

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