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Simple Regexp Syntax

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.

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

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