Handy Software Lab Products Knowlege Base

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 ;) ), handily and powerful tool.

We recommend You to play with regular expressions using RegExp Studio - it'll help You to understand main conceptions. Moreover, there are many predefined examples with comments included into repository of RE visual debugger.

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 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.

RegExpr Engine works with line separators as recommended at www.unicode.org:

"^" 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) doesn't match the empty string within the sequence \x0D\x0A, but matches the empty string within the sequence \x0A\x0D.

Multiline processing can be easily tuned for Your own purpose. You can use only Unix style separators \n or only DOS/Windows style \r\n or mix them together (as described above and used by default) or define Your own line separators!


Metacharacters - predefined classes

\w an alphanumeric character (including "_")
\W a non alphanumeric
\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 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 (after parsing You can find subexpression positions).

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

There are many ways to set up modifiers.
Any of these modifiers may be embedded within the regular expression itself using the (?...) construct.
Also, You can assign to appropriate RegExpr Engine properties (ModifierX for example to change /x, or ModifierStr to change all modifiers together).

i

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