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Diffstat (limited to 'srclib/pcre/doc/Tech.Notes')
| -rw-r--r-- | srclib/pcre/doc/Tech.Notes | 27 |
1 files changed, 19 insertions, 8 deletions
diff --git a/srclib/pcre/doc/Tech.Notes b/srclib/pcre/doc/Tech.Notes index 03904db3cc..f5ca280115 100644 --- a/srclib/pcre/doc/Tech.Notes +++ b/srclib/pcre/doc/Tech.Notes @@ -135,7 +135,7 @@ end of each byte. Back references --------------- -OP_REF is followed by a single byte containing the reference number. +OP_REF is followed by two bytes containing the reference number. Repeating character classes and back references @@ -163,11 +163,21 @@ Brackets and alternation A pair of non-capturing (round) brackets is wrapped round each expression at compile time, so alternation always happens in the context of brackets. + Non-capturing brackets use the opcode OP_BRA, while capturing brackets use OP_BRA+1, OP_BRA+2, etc. [Note for North Americans: "bracket" to some English speakers, including myself, can be round, square, curly, or pointy. Hence this usage.] +Originally PCRE was limited to 99 capturing brackets (so as not to use up all +the opcodes). From release 3.5, there is no limit. What happens is that the +first ones, up to EXTRACT_BASIC_MAX are handled with separate opcodes, as +above. If there are more, the opcode is set to EXTRACT_BASIC_MAX+1, and the +first operation in the bracket is OP_BRANUMBER, followed by a 2-byte bracket +number. This opcode is ignored while matching, but is fished out when handling +the bracket itself. (They could have all been done like this, but I was making +minimal changes.) + A bracket opcode is followed by two bytes which give the offset to the next alternative OP_ALT or, if there aren't any branches, to the matching KET opcode. Each OP_ALT is followed by two bytes giving the offset to the next one, @@ -191,8 +201,8 @@ appropriate. A subpattern with a bounded maximum repetition is replicated in a nested fashion up to the maximum number of times, with BRAZERO or BRAMINZERO before each replication after the minimum, so that, for example, (abc){2,5} is -compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 200-bracket limit does not -apply to these internally generated brackets. +compiled as (abc)(abc)((abc)((abc)(abc)?)?)?. The 99 and 200 bracket limits do +not apply to these internally generated brackets. Assertions @@ -202,9 +212,10 @@ Forward assertions are just like other subpatterns, but starting with one of the opcodes OP_ASSERT or OP_ASSERT_NOT. Backward assertions use the opcodes OP_ASSERTBACK and OP_ASSERTBACK_NOT, and the first opcode inside the assertion is OP_REVERSE, followed by a two byte count of the number of characters to move -back the pointer in the subject string. A separate count is present in each -alternative of a lookbehind assertion, allowing them to have different fixed -lengths. +back the pointer in the subject string. When operating in UTF-8 mode, the count +is a character count rather than a byte count. A separate count is present in +each alternative of a lookbehind assertion, allowing them to have different +fixed lengths. Once-only subpatterns @@ -219,7 +230,7 @@ Conditional subpatterns These are like other subpatterns, but they start with the opcode OP_COND. If the condition is a back reference, this is stored at the start of the -subpattern using the opcode OP_CREF followed by one byte containing the +subpattern using the opcode OP_CREF followed by two bytes containing the reference number. Otherwise, a conditional subpattern will always start with one of the assertions. @@ -239,4 +250,4 @@ the compiled data. Philip Hazel -February 2000 +August 2001 |