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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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> > | Related Links:- http://perldoc.perl.org/perlretut.html - Regular expressions tutorial
- http://www.perl.com/doc/manual/html/pod/perlre.html - Perl regular expressions
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## Regular Expressions |

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## Regular Expressions | ||||||||

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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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## Regular Expressions | ||||||||

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## IntroductionRegular expressions (REs), unlike simple queries, allow you to search for text which matches a particular pattern. | ||||||||

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## Searches with "and" combinations |

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> > | ## Regular Expressions
On this page:
## 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., | |||||||||||||||||||||||||||||||||||

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< < | Examples | ||||||||||||||||||||||||||||||||||

> > | ## Examples
## 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`
## AdvancedHere 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. | ||||||||||||||||||||||||||||||||||

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< < |
Here is stuff for our UNIX freaks:
\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. | ||||||||||||||||||||||||||||||||||

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> > | 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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< < | Regular expressions allow more specific queries then a simple query. | |||||||

> > | 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., | |||||||

Examples |

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> > | Regular expressions allow more specific queries then a simple query.
Here is stuff for our UNIX freaks:
\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. |

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