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Regular Expressions
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Related Topics: UserDocumentationCategory, VarSEARCH, FormattedSearch, QuerySearch  
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Regular Expressions
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Searches with "and" combinations  
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Use Google if your TWiki site is public. Example search for "form" and "template" at TWiki.org:  
Advanced  
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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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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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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 commandline interfaces found in operating systems such as Unix and MSDOS. REs are used by sophisticated search engines, as well as by many Unixbased languages and tools ( e.g.,  
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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 onecharacter 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 onecharacter regular expression that matches any character except NEWLINE. [string] A nonempty string of characters enclosed in square brackets is a onecharacter 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 onecharacter 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, [09] 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, []af] 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 onecharacter regular expression followed by `*' (an asterisk) is a regular expression that matches zero or more occurrences of the onecharacter 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 onecharacter 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 (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 commandline interfaces found in operating systems such as Unix and MSDOS. REs are used by sophisticated search engines, as well as by many Unixbased languages and tools ( e.g.,  
Examples 
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Examples
Here is stuff for our UNIX freaks:
\c A backslash (\) followed by any special character is a onecharacter 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 onecharacter regular expression that matches any character except NEWLINE. [string] A nonempty string of characters enclosed in square brackets is a onecharacter 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 onecharacter 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, [09] 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, []af] 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 onecharacter regular expression followed by `*' (an asterisk) is a regular expression that matches zero or more occurrences of the onecharacter 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 onecharacter 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. 