add example to count nb of seq in fastq with grep

source/Unix/search_filter.rst

@@ -57,6 +57,20 @@ Exercises
 * count the number of sequences in the file 'abcd.fa' (a multiple fasta file)
 * can we have the same approach with a fastq file?
 
+.. note::
+
+    We can count the number of + but be careful *+* may appear in quality line even in first position.
+    So we have to count the *+* which are alone on the line.
+
+    | The line **start** (anchor ^) and finish (anchor $) by *+*
+    | so our pattern is :code:`^+$`
+    | do not forget to protect your pattern by surrounding it by quotes 'pattern'
+    | The complete command line is:
+
+    .. code-block::
+
+        grep -c '^+$' my_super_fichier.fastq
+
 
 grep with Regular Expressions
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