CSVediting.md

@@ -143,10 +143,24 @@ P->CreateKernelCall<LineSpansKernel>(MatchedLineStarts, MatchedLineEnds, Matched
 If FilterByMask was used at this point, you would get the matched rows.   To delete
 the matched rows, the MatchedLineSpans must be negated (use a Pablo createNot operation).
 
+## Constraining Matches to a Column
+
+Matches can be constrained to a particular column.   The general method for
+matches within a row can be modified by using a column mask having 1 bits only
+within the column and supplying that as input to the regular expression
+matching process.   This will require a modification to the 
+options of the ICGrep kernel, to supply this as an option when
+calling the RE compiler.   When the ICGrep kernel is called,
+the mask must then be passed as the second
+parameter to the compileRE method of the RE compiler.
+```
+    Marker compileRE(RE * re, Marker initialMarkers);
+```
+If the marker stream returned by the RE_compiler has a one bit anywhere
+within the column, then a match is found.   
+
 ## Combining Masks
 
 If masks are computed to edit out both a column and a row, these can be combined with a Pablo
 createAnd operation and then one FilterByMask can be applied.
 
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