diff --git a/src/p_acs.cpp b/src/p_acs.cpp
index 6ccf7339d0..18ab347eba 100644
--- a/src/p_acs.cpp
+++ b/src/p_acs.cpp
@@ -4127,6 +4127,11 @@ enum EACSFunctions
 	ACSF_LineAttack,
 	ACSF_PlaySound,
 	ACSF_StopSound,
+	ACSF_strcmp,
+	ACSF_stricmp,
+	ACSF_StrLeft,
+	ACSF_StrRight,
+	ACSF_StrMid,
 
 	// ZDaemon
 	ACSF_GetTeamScore = 19620,	// (int team)
@@ -4951,6 +4956,75 @@ doplaysound:			if (!looping)
 			}
 			break;
 
+		case ACSF_strcmp:
+		case ACSF_stricmp:
+			if (argCount >= 2)
+			{
+				const char *a, *b;
+				a = FBehavior::StaticLookupString(args[0]);
+				b = FBehavior::StaticLookupString(args[1]);
+
+				// Don't crash on invalid strings.
+				if (a == NULL) a = "";
+				if (b == NULL) b = "";
+
+				if (argCount > 2)
+				{
+					int n = args[2];
+					return (funcIndex == ACSF_strcmp) ? strncmp(a, b, n) : strnicmp(a, b, n);
+				}
+				else
+				{
+					return (funcIndex == ACSF_strcmp) ? strcmp(a, b) : stricmp(a, b);
+				}
+			}
+			break;
+
+		case ACSF_StrLeft:
+		case ACSF_StrRight:
+			if (argCount >= 2)
+			{
+				const char *oldstr = FBehavior::StaticLookupString(args[0]);
+				if (oldstr == NULL || *oldstr == '\0')
+				{
+					return GlobalACSStrings.AddString("");
+				}
+				size_t oldlen = strlen(oldstr);
+				size_t newlen = args[1];
+
+				if (oldlen < newlen)
+				{
+					newlen = oldlen;
+				}
+				FString newstr(funcIndex == ACSF_StrLeft ? oldstr : oldstr + oldlen - newlen, newlen);
+				return GlobalACSStrings.AddString(newstr);
+			}
+			break;
+
+		case ACSF_StrMid:
+			if (argCount >= 3)
+			{
+				const char *oldstr = FBehavior::StaticLookupString(args[0]);
+				if (oldstr == NULL || *oldstr == '\0')
+				{
+					return GlobalACSStrings.AddString("");
+				}
+				size_t oldlen = strlen(oldstr);
+				size_t pos = args[1];
+				size_t newlen = args[2];
+
+				if (pos >= oldlen)
+				{
+					return GlobalACSStrings.AddString("");
+				}
+				if (pos + newlen > oldlen || pos + newlen < pos)
+				{
+					newlen = oldlen - pos;
+				}
+				return GlobalACSStrings.AddString(FString(oldstr + pos, newlen));
+			}
+			break;
+
 		default:
 			break;
 	}