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libregexp: treat empty strings as valid regexp

assumed behavior in awk (// and !// are valid patterns)
and other tools.

--- a/sys/man/6/regexp

+++ b/sys/man/6/regexp

@@ -91,6 +91,8 @@

 matches the beginning of a line;

 .L $

 matches the end of the line.

+An empty regular expression behaves equivalently to

+.LR ^ .

 .PP

 The 

 .B REP

--- a/sys/src/libregexp/regcomp.c

+++ b/sys/src/libregexp/regcomp.c

@@ -164,6 +164,10 @@

 	int regstrlen, maxthr;

 	regstrlen = utflen(regstr);

+	if(regstrlen == 0){

+		regstr = "^";

+		regstrlen = 1;

+	}

 	initplex(&plex, regstr, lit);

 	plex.nodes = calloc(sizeof(*plex.nodes), regstrlen*2);

 	if(plex.nodes == nil)

--- a/sys/src/libregexp/test/basic.rc

+++ b/sys/src/libregexp/test/basic.rc

@@ -42,6 +42,10 @@

 #  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE

 #  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

+M	'^'	'abracadabracadabra'	'(0,0)'

+M	'^'	''	'(0,0)'

+M	''	'abracadabracadabra'	'(0,0)'

+M	''	''	'(0,0)'

 M	'abracadabra$'	'abracadabracadabra'	'(7,18)'

 M	'a...b'	'abababbb'	'(2,7)'

 M	'XXXXXX'	'..XXXXXX'	'(2,8)'

-- 

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