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> | Regular Expressions
Introduction | | | Regular expressions (REs), unlike simple queries, allow you to search for text which matches a particular pattern.
REs are similar to (but more poweful than) the "wildcards" used in the command-line interfaces found in operating systems such as Unix and MS-DOS. REs are used by sophisticated search engines, as well as by many Unix-based languages and tools ( e.g., awk, grep, lex, perl, and sed ). | |
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< | Examples | >
> | Examples
compan(y|ies) | Search for company, companies |
(peter|paul) | Search for peter, paul |
bug* | Search for bug, bugs, bugfix |
[Bb]ag | Search for Bag, bag |
b[aiueo]g | Second letter is a vowel. Matches bag, bug, big |
b.g | Second letter is any letter. Matches also b&g |
[a-zA-Z] | Matches any one letter (but not a number or a symbol) |
[^0-9a-zA-Z] | Matches any symbol (but not a number or a letter) |
[A-Z][A-Z]* | Matches one or more uppercase letters |
[0-9]{3}-[0-9]{2}-[0-9]{4} | US social security number, e.g. 123-45-6789 |
PNG;Chart | Search for topics containing the words PNG and Chart. This is not a regular expression! But a useful facility that is enabled when regular expression searching is enabled. |
Searches with "and" combinations
- TWiki extends the regular expressions with an and search. The delimiter is a semicolon
;. Example search for "form" and "template": form;template
- Use Google if your TWiki site is public. Example search for "form" and "template" at TWiki.org:
site:twiki.org +form +template
Advanced
Here is stuff for our UNIX freaks: (copied from 'man egrep')
A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions.
The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any metacharacter with special meaning may be quoted by preceding it with a backslash.
A bracket expression is a list of characters enclosed by [ and ]. It matches any single character in that list; if the first character of the list is the caret ^ then it matches any character not in the list. For example, the regular expression [0123456789] matches any single digit.
Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, inclusive, using the locale's collating sequence and character set. For example, in the default C locale, [a-d] is equivalent to [abcd]. Many locales sort characters in dictionary order, and in these locales [a-d] is typically not equivalent to [abcd]; it might be equivalent to [aBbCcDd], for example.
Finally, certain named classes of characters are predefined within bracket expressions, as follows. Their names are self explanatory, and they are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:]. For example, [[:alnum:]] means [0-9A-Za-z], except the latter form depends upon the C locale and the ASCII character encoding, whereas the former is independent of locale and character set. (Note that the brackets in these class names are part of the symbolic names, and must be included in addition to the brackets delimiting the bracket list.) Most metacharacters lose their special meaning inside lists. To include a literal ] place it first in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a literal - place it last.
The period . matches any single character. The symbol \w is a synonym for [[:alnum:]] and \W is a synonym for [^[:alnum]].
The caret ^ and the dollar sign $ are metacharacters that respectively match the empty string at the beginning and end of a line. The symbols \< and \> respectively match the empty string at the beginning and end of a word. The symbol \b matches the empty string at the edge of a word, and \B matches the empty string provided it's not at the edge of a word.
A regular expression may be followed by one of several repetition operators:
| ? | The preceding item is optional and matched at most once. |
| * | The preceding item will be matched zero or more times. |
| + | The preceding item will be matched one or more times. |
| {n} | The preceding item is matched exactly n times. |
| {n,} | The preceding item is matched n or more times. |
| {n,m} | The preceding item is matched at least n times, but not more than m times. |
Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated subexpressions.
Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string matching either subexpression. | | | | |
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compan(y|ies)
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Search for company , companies
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(peter|paul)
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Search for peter , paul
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bug*
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Search for bug , bugs , bugfix
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[Bb]ag
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Search for Bag , bag
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b[aiueo]g
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Second letter is a vowel. Matches bag , bug , big
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b.g
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Second letter is any letter. Matches also b&g
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[a-zA-Z]
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Matches any one letter (not a number and a symbol)
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[^0-9a-zA-Z]
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Matches any symbol (not a number or a letter)
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[A-Z][A-Z]*
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Matches one or more uppercase letters
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[0-9][0-9][0-9]-[0-9][0-9]-
[0-9][0-9][0-9][0-9]
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US social security number, e.g. 123-45-6789
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Here is stuff for our UNIX freaks:
(copied from 'man grep')
\c A backslash (\) followed by any special character is a
one-character regular expression that matches the spe-
cial character itself. The special characters are:
+ `.', `*', `[', and `\' (period, asterisk,
left square bracket, and backslash, respec-
tively), which are always special, except
when they appear within square brackets ([]).
+ `^' (caret or circumflex), which is special
at the beginning of an entire regular expres-
sion, or when it immediately follows the left
of a pair of square brackets ([]).
+ $ (currency symbol), which is special at the
end of an entire regular expression.
. A `.' (period) is a one-character regular expression
that matches any character except NEWLINE.
[string]
A non-empty string of characters enclosed in square
brackets is a one-character regular expression that
matches any one character in that string. If, however,
the first character of the string is a `^' (a circum-
flex or caret), the one-character regular expression
matches any character except NEWLINE and the remaining
characters in the string. The `^' has this special
meaning only if it occurs first in the string. The `-'
(minus) may be used to indicate a range of consecutive
ASCII characters; for example, [0-9] is equivalent to
[0123456789]. The `-' loses this special meaning if it
occurs first (after an initial `^', if any) or last in
the string. The `]' (right square bracket) does not
terminate such a string when it is the first character
within it (after an initial `^', if any); that is,
[]a-f] matches either `]' (a right square bracket ) or
one of the letters a through f inclusive. The four
characters `.', `*', `[', and `\' stand for themselves
within such a string of characters.
The following rules may be used to construct regular expres-
sions:
* A one-character regular expression followed by `*' (an
asterisk) is a regular expression that matches zero or
more occurrences of the one-character regular expres-
sion. If there is any choice, the longest leftmost
string that permits a match is chosen.
^ A circumflex or caret (^) at the beginning of an entire
regular expression constrains that regular expression
to match an initial segment of a line.
$ A currency symbol ($) at the end of an entire regular
expression constrains that regular expression to match
a final segment of a line.
* A regular expression (not just a one-
character regular expression) followed by `*'
(an asterisk) is a regular expression that
matches zero or more occurrences of the one-
character regular expression. If there is
any choice, the longest leftmost string that
permits a match is chosen.
+ A regular expression followed by `+' (a plus
sign) is a regular expression that matches
one or more occurrences of the one-character
regular expression. If there is any choice,
the longest leftmost string that permits a
match is chosen.
? A regular expression followed by `?' (a ques-
tion mark) is a regular expression that
matches zero or one occurrences of the one-
character regular expression. If there is
any choice, the longest leftmost string that
permits a match is chosen.
| Alternation: two regular expressions
separated by `|' or NEWLINE match either a
match for the first or a match for the
second.
() A regular expression enclosed in parentheses
matches a match for the regular expression.
The order of precedence of operators at the same parenthesis
level is `[ ]' (character classes), then `*' `+' `?'
(closures),then concatenation, then `|' (alternation)and
NEWLINE.
| >
> | Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole subexpression may be enclosed in parentheses to override these precedence rules. | | | | |
>
> | The backreference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression.
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