Perl Regular Expression Debug Support
Perl Regular Expressions are supported beginning with SAS®9.
Information about how a regexp is compiled or the steps taken when
matching a regexp against a character value could be valuable when a
regexp does not do what you intended.
Perl produces regexp debugging output by using the -Dr switch on the
command line or by using use re 'debug' withing a program.
The data step provides this functionality with the PRXDEBUG call
routine. CALL PRXDEBUG(1) turns on debug output and CALL
PRXDEBUG(0) turns off debug output. All debug output is sent to
the SAS log.
This paper will present an example of and describe Perl debug output.
All of this material is taken from the perldebguts man page or at
http://search.cpan.org/~gsar/perl-5.6.1/pod/perldebguts.pod#Debugging_regular_expressions.
Perl debug output is turned on between the two calls to PRXDEBUG in
the program below. The output produced by PRXPARSE and PRXMATCH will
be discussed in the following sections.
data _null_;
call prxdebug(1);
putlog 'PRXPARSE:';
re = prxparse('/[bc]d(ef*g)+h[ij]k$/');
putlog 'PRXMATCH:';
pos = prxmatch(re, 'abcdefg__gh__');
call prxdebug(0);
run;
Compiling REx `[bc]d(ef*g)+h[ij]k$'
size 41 first at 1
rarest char g at 0
rarest char d at 0
1: ANYOF[bc](10)
10: EXACT <d>(12)
12: CURLYX[0] {1,32767}(26)
14: OPEN1(16)
16: EXACT <e>(18)
18: STAR(21)
19: EXACT <f>(0)
21: EXACT <g>(23)
23: CLOSE1(25)
25: WHILEM[1/1](0)
26: NOTHING(27)
27: EXACT <h>(29)
29: ANYOF[ij](38)
38: EXACT <k>(40)
40: EOL(41)
41: END(0)
anchored `de' at 1 floating `gh' at 3..2147483647 (checking floating)
stclass `ANYOF[bc]' minlen 7
The first line shows the pre-compiled form of the regex. The second
shows the size of the compiled form (in arbitrary units, usually
4-byte words) and the label id of the first node that does a
match.
The last line (split into two lines above) contains optimizer
information. In the example shown, the optimizer found that the match
should contain a substring de
at offset 1, plus substring gh
at some offset between 3 and infinity. Moreover, when checking for
these substrings (to abandon impossible matches quickly), Perl will check
for the substring gh
before checking for the substring de
. The
optimizer may also use the knowledge that the match starts (at the
first
id) with a character class, and the match cannot be
shorter than 7 chars.
The fields of interest which may appear in the last line are
anchored
STRING at
POS
-
floating
STRING at
POS1..POS2
-
See above.
matching floating/anchored
-
Which substring to check first.
minlen
-
The minimal length of the match.
stclass
TYPE
-
Type of first matching node.
noscan
-
Don't scan for the found substrings.
isall
-
Means that the optimizer info is all that the regular
expression contains, and thus one does not need to enter the regex engine at
all.
GPOS
-
Set if the pattern contains
\G
.
plus
-
Set if the pattern starts with a repeated char (as in
x+y
).
implicit
-
Set if the pattern starts with
.*
.
with eval
-
Set if the pattern contain eval-groups, such as
(?{ code })
and
(??{ code })
.
anchored(TYPE)
-
If the pattern may match only at a handful of places, (with
TYPE
being BOL
, MBOL
, or GPOS
. See the table below.
If a substring is known to match at end-of-line only, it may be
followed by $
, as in floating `k'$
.
The optimizer-specific info is used to avoid entering (a slow) regex
engine on strings that will not definitely match. If isall
flag
is set, a call to the regex engine may be avoided even when the optimizer
found an appropriate place for the match.
The rest of the output contains the list of nodes of the compiled
form of the regex. Each line has format
id: TYPE OPTIONAL-INFO (next-id)
Types of nodes
Here are the possible types, with short descriptions:
# TYPE arg-description [num-args] [longjump-len] DESCRIPTION
# Exit points
END no End of program.
SUCCEED no Return from a subroutine, basically.
# Anchors:
BOL no Match "" at beginning of line.
MBOL no Same, assuming multiline.
SBOL no Same, assuming singleline.
EOS no Match "" at end of string.
EOL no Match "" at end of line.
MEOL no Same, assuming multiline.
SEOL no Same, assuming singleline.
BOUND no Match "" at any word boundary
BOUNDL no Match "" at any word boundary
NBOUND no Match "" at any word non-boundary
NBOUNDL no Match "" at any word non-boundary
GPOS no Matches where last m//g left off.
# [Special] alternatives
ANY no Match any one character (except newline).
SANY no Match any one character.
ANYOF sv Match character in (or not in) this class.
ALNUM no Match any alphanumeric character
ALNUML no Match any alphanumeric char in locale
NALNUM no Match any non-alphanumeric character
NALNUML no Match any non-alphanumeric char in locale
SPACE no Match any whitespace character
SPACEL no Match any whitespace char in locale
NSPACE no Match any non-whitespace character
NSPACEL no Match any non-whitespace char in locale
DIGIT no Match any numeric character
NDIGIT no Match any non-numeric character
# BRANCH The set of branches constituting a single choice are hooked
# together with their "next" pointers, since precedence prevents
# anything being concatenated to any individual branch. The
# "next" pointer of the last BRANCH in a choice points to the
# thing following the whole choice. This is also where the
# final "next" pointer of each individual branch points; each
# branch starts with the operand node of a BRANCH node.
#
BRANCH node Match this alternative, or the next...
# BACK Normal "next" pointers all implicitly point forward; BACK
# exists to make loop structures possible.
# not used
BACK no Match "", "next" ptr points backward.
# Literals
EXACT sv Match this string (preceded by length).
EXACTF sv Match this string, folded (prec. by length).
EXACTFL sv Match this string, folded in locale (w/len).
# Do nothing
NOTHING no Match empty string.
# A variant of above which delimits a group, thus stops optimizations
TAIL no Match empty string. Can jump here from outside.
# STAR,PLUS '?', and complex '*' and '+', are implemented as circular
# BRANCH structures using BACK. Simple cases (one character
# per match) are implemented with STAR and PLUS for speed
# and to minimize recursive plunges.
#
STAR node Match this (simple) thing 0 or more times.
PLUS node Match this (simple) thing 1 or more times.
CURLY sv 2 Match this simple thing {n,m} times.
CURLYN no 2 Match next-after-this simple thing
# {n,m} times, set parens.
CURLYM no 2 Match this medium-complex thing {n,m} times.
CURLYX sv 2 Match this complex thing {n,m} times.
# This terminator creates a loop structure for CURLYX
WHILEM no Do curly processing and see if rest matches.
# OPEN,CLOSE,GROUPP ...are numbered at compile time.
OPEN num 1 Mark this point in input as start of #n.
CLOSE num 1 Analogous to OPEN.
REF num 1 Match some already matched string
REFF num 1 Match already matched string, folded
REFFL num 1 Match already matched string, folded in loc.
# grouping assertions
IFMATCH off 1 2 Succeeds if the following matches.
UNLESSM off 1 2 Fails if the following matches.
SUSPEND off 1 1 "Independent" sub-regex.
IFTHEN off 1 1 Switch, should be preceded by switcher .
GROUPP num 1 Whether the group matched.
# Support for long regex
LONGJMP off 1 1 Jump far away.
BRANCHJ off 1 1 BRANCH with long offset.
# The heavy worker
EVAL evl 1 Execute some Perl code.
# Modifiers
MINMOD no Next operator is not greedy.
LOGICAL no Next opcode should set the flag only.
# This is not used yet
RENUM off 1 1 Group with independently numbered parens.
# This is not really a node, but an optimized away piece of a "long" node.
# To simplify debugging output, we mark it as if it were a node
OPTIMIZED off Placeholder for dump. |
|
First of all, when doing a match, one may get no run-time output even
if debugging is enabled. This means that the regex engine was never
entered and that all of the job was therefore done by the optimizer.
Guessing start of match, REx `[bc]d(ef*g)+h[ij]k$' against `abcdefg__gh__'...
Found floating substr `gh' at offset 9...
Found anchored substr `de' at offset 3...
Starting position does not contradict /^/m...
Does not contradict STCLASS...
Guessed: match at offset 2
Matching REx `[bc]d(ef*g)+h[ij]k$' against `cdefg__gh__'
Setting an EVAL scope, savestack=0
2 <ab> <cdefg__gh_> | 1: ANYOF[bc]
3 <abc> <defg__gh_> | 10: EXACT <d>
4 <abcd> <efg__gh_> | 12: CURLYX[0] {1,32767}
4 <abcd> <efg__gh_> | 25: WHILEM[1/1]
0 out of 1..32767 cc=3dcfca4
4 <abcd> <efg__gh_> | 14: OPEN1
4 <abcd> <efg__gh_> | 16: EXACT <e>
5 <abcde> <fg__gh_> | 18: STAR
EXACT <f> can match 1 times out of 32767...
Setting an EVAL scope, savestack=0
6 <bcdef> <g__gh__> | 21: EXACT <g>
7 <bcdefg> <__gh__> | 23: CLOSE1
7 <bcdefg> <__gh__> | 25: WHILEM[1/1]
1 out of 1..32767 cc=3dcfca4
Setting an EVAL scope, savestack=9
7 <bcdefg> <__gh__> | 14: OPEN1
7 <bcdefg> <__gh__> | 16: EXACT <e>
failed...
restoring \1 to 4(4)..7
failed, try continuation...
7 <bcdefg> <__gh__> | 26: NOTHING
7 <bcdefg> <__gh__> | 27: EXACT <h>
failed...
failed...
failed...
failed...
failed...
Match failed
The most significant information in the output is about the particular node
of the compiled regex that is currently being tested against the target string.
The format of these lines is
STRING-OFFSET <PRE-STRING> <POST-STRING> |ID: TYPE
The TYPE info is indented with respect to the backtracking level.
Other incidental information appears interspersed within.
Your Turn
The developers, testers and documentation folk that bring you Base SAS
Software are very excited about the potential of these new capabilities of
the SAS System. You can send electronic mail to
Base.Research@sas.com with your
comments.