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$XFree86: xc/programs/xedit/lisp/re/README,v 1.3 2002/09/23 01:25:41 paulo Exp $
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This is a small regex library for fast matching tokens in text. It was built
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to be used by xedit and it's syntax highlight code. It is not compliant with
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IEEE Std 1003.2, but is expected to be used where very fast matching is
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required, and exotic patterns will not be used.
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To understand what kind of patterns this library is expected to be used with,
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see the file <XRoot>xc/programs/xedit/lisp/modules/progmodes/c.lsp and some
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samples in the file tests.txt, with comments for patterns that will not work,
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or may give incorrect results.
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The library is not built upon the standard regex library by Henry Spencer,
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but is completely written from scratch, but it's syntax is heavily based on
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that library, and the only reason for it to exist is that unfortunately
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the standard version does not fit the requirements needed by xedit.
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Anyways, I would like to thanks Henry for his regex library, it is a really
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Small description of understood tokens:
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------------------------------------------------------------------------
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. Any character (won't match newline if compiled with RE_NEWLINE)
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\w Any word letter (shortcut to [a-zA-Z0-9_]
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\W Not a word letter (shortcut to [^a-zA-Z0-9_]
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\D Not a decimal number
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\l A lower case letter
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\u An upper case letter
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\c A control character, currently the range 1-32 (minus tab)
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\C Not a control character
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\O Not an octal number
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\X Not an hexadecimal number
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\< Beginning of a word (matches an empty string)
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\> End of a word (matches an empty string)
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^ Beginning of a line (matches an empty string)
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$ End of a line (matches an empty string)
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[...] Matches one of the characters inside the brackets
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ranges are specified separating two characters with "-".
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If the first character is "^", matches only if the
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character is not in this range. To add a "]" make it
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the first character, and to add a "-" make it the last.
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\1 to \9 Backreference, matches the text that was matched by a group,
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that is, text that was matched by the pattern inside
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------------------------------------------------------------------------
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() Any pattern inside works as a backreference, and is also
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used to group patterns.
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| Alternation, allows choosing different possibilities, like
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character ranges, but allows patterns of different lengths.
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------------------------------------------------------------------------
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<re>* <re> may occur any number of times, including zero
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<re>+ <re> must occur at least once
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<re>? <re> is optional
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<re>{<e>} <re> must occur exactly <e> times
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<re>{<n>,} <re> must occur at least <n> times
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<re>{,<m>} <re> must not occur more than <m> times
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<re>{<n>,<m>} <re> must occur at least <n> times, but no more than <m>
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Note that "." is a special character, and when used with a repetition
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operator it changes completely its meaning. For example, ".*" matches
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anything up to the end of the input string (unless the pattern was compiled
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with RE_NEWLINE, in that case it will match anything, but a newline).
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o Only minimal matches supported. The engine has only one level "backtracking",
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so, it also only does minimal matches to allow backreferences working
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properly, and to avoid failing to match depending on the input.
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o Only one level "grouping", for example, with the pattern:
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If "abc" is anywhere in the input, it will be in "\1", but there will
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not exist a "\2" for "b".
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o Some "special repetitions" were not implemented, these are:
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o Some patterns will never match, for example:
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Since "\w*" already includes all possible matches of "\d", "\d" will
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only be tested when "\w*" failed. There are no plans to make such
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Some of these limitations may be worked on future versions of the library,
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but this is not what the library is expected to do, and, adding support for
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correct handling of these would probably make the library slower, what is
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not the reason of it to exist in the first time.
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If you need "true" regex than this library is not for you, but if all
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you need is support for very quickly finding simple patterns, than this
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library can be a very powerful tool, on some patterns it can run more
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than 200 times faster than "true" regex implementations! And this is
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the reason it was written.
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Send comments and code to me (paulo@XFree86.Org) or to the XFree86
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xedit/lisp/re/README
