diff --git a/README.md b/README.md
index 0965125..8232754 100644
--- a/README.md
+++ b/README.md
@@ -44,4 +44,30 @@ $HOSTNAME : current hostname
 var="some string"
 echo ${#var}
 # 11
+```
+
+### replace
+
+```
+foo=tachchen
+echo ${foo/a/-}
+# t-chchen                                                                                                                        /0,0s
+echo ${foo//c/-}
+# ta-h-hen
+```
+
+## Math
+
+### prime factors
+
+```
+factor 50
+# 50: 2 5 5
+```
+
+## Sed
+
+```
+sed 1d filename         # remove first line
+sed 1,100d filename     # remove line 1-100
 ```
\ No newline at end of file