diff options
Diffstat (limited to 'doc/pegdocs.txt')
| -rwxr-xr-x | doc/pegdocs.txt | 21 |
1 files changed, 12 insertions, 9 deletions
diff --git a/doc/pegdocs.txt b/doc/pegdocs.txt index 943acc545..e2c228cf3 100755 --- a/doc/pegdocs.txt +++ b/doc/pegdocs.txt @@ -60,6 +60,8 @@ notation meaning ``_`` Any Unicode character: If there is an UTF-8 character ahead, consume it and indicate success. Otherwise indicate failure. +``@E`` Search: Shorthand for ``(!E .)* E``. (Search loop for the + pattern `E`.) ``A <- E`` Rule: Bind the expression `E` to the *nonterminal symbol* `A`. **Left recursive rules are not possible and crash the matching engine.** @@ -118,14 +120,15 @@ The PEG parser implements this grammar (written in PEG syntax):: "'" ("\\" . / [^'])* "'") builtin <- "\\" identifier / [^\13\10] - comment <- '#' !\n* \n + comment <- '#' @ \n ig <- (\s / comment)* # things to ignore rule <- identifier \s* "<-" expr ig identNoArrow <- identifier !(\s* "<-") - primary <- (ig '&' / ig '!')* ((ig identNoArrow / ig charset / ig stringlit - / ig builtin / ig '.' / ig '_' - / (ig "(" expr ig ")")) + primary <- (ig '&' / ig '!' / ig '@')* + ((ig identNoArrow / ig charset / ig stringlit + / ig builtin / ig '.' / ig '_' + / (ig "(" expr ig ")")) (ig '?' / ig '*' / ig '+')*) # Concatenation has higher priority than choice: @@ -153,16 +156,16 @@ Determine the ``#include``'ed files of a C file: .. code-block:: nimrod for line in lines("myfile.c"): if line =~ peg"""s <- ws '#include' ws '"' {[^"]+} '"' ws - comment <- '/*' (!'*/' . )* '*/' / '//' .* - ws <- (comment / \s+)* """: + comment <- '/*' @ '*/' / '//' .* + ws <- (comment / \s+)* """: echo matches[0] PEG vs regular expression ------------------------- -As a regular expression ``\[.*\]`` maches longest possible text between ``'['`` -and ``']'``. As a PEG it never matches anything, because a PEG is +As a regular expression ``\[.*\]`` matches the longest possible text between +``'['`` and ``']'``. As a PEG it never matches anything, because a PEG is deterministic: ``.*`` consumes the rest of the input, so ``\]`` never matches. -As a PEG this needs to be written as: ``\[ ( !\] . )* \]`` +As a PEG this needs to be written as: ``\[ ( !\] . )* \]`` (or ``\[ @ \]``). Note that the regular expression does not behave as intended either: ``*`` should not be greedy, so ``\[.*?\]`` should be used. |