342 lines
8.2 KiB
Plaintext
342 lines
8.2 KiB
Plaintext
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archie uses ed(1) regular expressions in a number of
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commands.
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A regular expression, on the one hand, is a string like
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any other; a sequence of characters. On the other
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hand, special characters within the string have certain
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functions which make regular expressions useful when
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trying to match portions of other strings. In the fol-
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lowing discussion and examples, a string containing a
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regular expression will be called the ``pattern'', and
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the string against which it is to be matched is called
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the ``reference string''.
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Regular expressions allow one to search for "all strings
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ending with the letters 'ize'" or "all strings beginning
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with a number between 1 and 3 and ending in a comma".
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In order to accomplish this, regular expressions co-opt
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the use of some characters to have special meaning.
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They also provide for these characters to lose their
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special meaning if the user so desires. The rules for
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regular expresssion are
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c Any character c matches itself unless it has been
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assigned other special meaning as listed below. Most
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special characters can be escaped (made to lose its
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special meaning), by placing the character '\' in front
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of it. This doesn't apply to '{' which is non-special
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until it is escaped. Thus although '*' normally has
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special meaning the string '\*' matches itself.
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Example:
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The pattern
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acdef
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matches
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s83acdeffff or acdefsecs or acdefsecs
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but not
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accdef or aacde1f
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That is it will any string that contains ``acdef'' any-
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where in the reference string.
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Example:
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Normally the characters '*' and '$' are special,
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but the pattern
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a\*bse\$
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acts as above. That is any reference string containing
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``*abse$'' as a substring will be flagged as a match.
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. A period matches any character except the newline
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character. This is known as the wildcard character.
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Example:
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The pattern
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....
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will match any 4 characters in the reference string,
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except a newline character.
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^ If `^' appears at the begining of the pattern then it
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is said to ``anchor'' the match to the beginning of the
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line. That is, the reference string must start with the
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pattern following the `^'. If this character appears
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anywhere else other than at the beginning of the line,
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then it is no longer considered special, and matches
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itself as any non-special character would. Similarly if
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it starts a string but is escaped, it matches itself.
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Example:
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The pattern
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^efghi
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Will match
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efghi or efghijlk
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but not
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abcefghi
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That is the pattern will match only those reference
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strings starting with ``efghi''. Just containing the
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substring is not sufficient.
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$ Occurring at the end of the pattern, this character
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``anchors'' the pattern to the end of the line (refer-
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ence string). A '$' occurring anywhere else in the pat-
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tern is regarded as a non-special. Similarly if it is
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at the end of the pattern but is escaped, it is non-
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special.
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Example:
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The pattern
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efghi$
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Will match
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efghi or abcdefghi
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but not
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efghijkl
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That is the pattern will match only those reference
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strings ending with ``efghi''. Just containing the sub-
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string is not sufficient.
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\< This sequence in the pattern causes the one character
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regular expression following it only to match something
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at the beginning of a word: the beginning of a line or
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just before a letter, digit or underline character, or
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just after a charcter which is not one of these.
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Example:
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The pattern
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\<abc
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would match the last 'abc' in the reference string
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@hijabc#+abc
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but not the first since the first 'abc' did not start
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on a ``word'' boundary.
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\> Constrains the one-character regular expression fol-
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lowing it to be at the end of a ``word'' as defined
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above.
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[string]
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One or more characters within square brackets. This
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pattern matches any single character within the brack-
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ets. The caret, '^', has a special meaning if it is the
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first character in the series: the pattern will match
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any character other than one in the list.
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Example:
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The pattern
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[^abc]
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Will match any character except 'a', 'b' or 'c'.
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To match a right bracket, ']', in the list it must be
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put first:
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[]ab01]
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For a caret, '^', in the list it can appear anywhere
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but first.
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In
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[ab^01]
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the caret loses its special meaning.
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The '-' character is special within square brackets. It
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is interpreted as a range of characters (in the ASCII
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character set) and will match any single character
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within that range. '[a-z]' matches any lower case
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letter. The '-' can be made non special by placing it
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first or last within the square brackets.
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The characters '$', '*' and '.' are not special within
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square brackets.
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Example:
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The pattern
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[ab01]
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matches a single occurence of a character from the set
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'a', 'b', '0', '1'.
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Example:
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The pattern
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[^ab01]
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will match any single character other than 'a', 'b',
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'0', '1'.
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Example :
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The pattern
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[a0-9b]
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which matches one of 'a', 'b' or a digit between 0 and
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9 inclusive.
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Example :
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The pattern
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[^a0-9b.$]
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means any single character not 'a', 'b' '.' , '$' or a
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digit between 0 and 9 inclusive.
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* An asterisk following a regular expression in the pat-
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tern has the effect of matching zero or more
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occurrences of that expression.
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Example:
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The pattern
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a*
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means zero or more occurrences of the character 'a'.
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Example:
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The pattern
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[A-Z]*
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means zero or more occurrences of the upper case alpha-
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bet.
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\{m\}
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\{m,\}
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\{m,n\}
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A one-character regular expression followed by one of
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the three of these constructions causes a range of
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occurrences of that regular expression to be matched.
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If it is followed by \{m\} where m is a non-negative
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integer between 0 and 255 (inclusive), then exactly m
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occurrences of that regular expression are matched. If
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followed by \{m,\}, then at least m occurrences are
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matched. Finally, if it is followed by \{m,n\} (where
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n is a non-negative integer between 0 and 255 and where
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n > m), then between m and n occurrences of the expres-
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sion are matched.
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Example:
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The pattern
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ab\{3\}
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would match any substring in the reference string of an
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'a' followed by exactly 3 'b's.
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Example:
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The pattern
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ab\{3,\}
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would match any substring in the reference string of an
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'a' followed by at least 3 'b's.
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Example:
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The pattern
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ab\{3,5\}
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would match any substring in the reference string of an
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'a' followed by at least 3 but at most 5 'b's.
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Common Problems with Regular Expression
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(1) When matching a substring it is not necessary to use
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the wildcard character to match the part of the refer-
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ence string preceeding and following the substring.
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Example:
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The pattern
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abcd
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will match any reference string containing this pat-
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tern. It is not necessary to use
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.*abcd.*
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as the pattern.
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(2) In order to constrain a pattern to the entire reference
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pattern, use the the construction:
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^pattern$
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(3) The easiest way to obtain case insensitivity in a regu-
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lar expression is to use the '[]' operator. For exam-
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ple, a pattern to match the word ``hello'' regarless of
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the case of the letters would be:
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[Hh][Ee][Ll][Ll][Oo]
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