Add regex error checking, and display a message to the user when their regular expression has a syntax error. Additionally, use a non-capturing group to surround the user input when `Match whole word` is enabled in case alternates are being used. Finally, add a safety check to highlighting to avoid an infinite loop when zero-length matches happen.

src/SMAPI.Web/Views/LogParser/Index.cshtml

@@ -406,6 +406,12 @@ else if (log?.IsValid == true)
                             <span role="button" v-bind:class="{ active: !filterInsensitive }" v-on:click="toggleFilterInsensitive" title="Match exact capitalization only.">aA</span>
                             <span role="button" v-bind:class="{ active: filterUseWord, 'whole-word': true }" v-on:click="toggleFilterWord" title="Match whole word only."><i>“ ”</i></span>
                             <span role="button" v-bind:class="{ active: shouldHighlight }" v-on:click="toggleHighlight" title="Highlight matches in the log text.">HL</span>
+                            <div
+                                v-if="filterError"
+                                class="filter-error"
+                            >
+                                {{ filterError }}
+                            </div>
                         </div>
                         <filter-stats
                             v-bind:start="start"

src/SMAPI.Web/wwwroot/Content/css/log-parser.css

@@ -187,6 +187,11 @@ table caption {
     margin-top: 0.5em;
 }
 
+#filters .filter-error {
+    color: #880000;
+}
+
+#filters .filter-error,
 #filters .stats {
     margin-top: 0.5em;
     font-size: 0.75em;
@@ -210,7 +215,7 @@ table caption {
 
 @media (max-width: 1019px) {
     #filters:not(.sticky) {
-        width: calc(100vw - 3em);
+        width: calc(100vw - 5em);
     }
 
     #filters {

src/SMAPI.Web/wwwroot/Content/js/log-parser.js

@@ -247,7 +247,8 @@ smapi.logParser = function (state) {
     // add the properties we're passing to Vue
     state.totalMessages = state.messages.length;
     state.filterText = "";
-    state.filterRegex = "";
+    state.filterRegex = null;
+    state.filterError = null;
     state.showContentPacks = true;
     state.useHighlight = true;
     state.useRegex = false;
@@ -506,6 +507,12 @@ smapi.logParser = function (state) {
                     }
 
                     index = match.index + match[0].length;
+
+                    // In the event of a zero-length match, forcibly increment
+                    // the last index of the regular expression to ensure we
+                    // aren't stuck in an infinite loop.
+                    if (match[0].length == 0)
+                        filter.lastIndex++;
                 }
 
                 // Add any trailing text after the last match was found.
@@ -857,14 +864,30 @@ smapi.logParser = function (state) {
                     if (!text || !text.length) {
                         this.filterText = "";
                         this.filterRegex = null;
+                        this.filterError = null;
                     }
                     else {
                         if (!state.useRegex)
                             text = helpers.escapeRegex(text);
-                        this.filterRegex = new RegExp(
-                            state.useWord ? `\\b${text}\\b` : text,
-                            state.useInsensitive ? "ig" : "g"
-                        );
+
+                        const flags = state.useInsensitive ? "ig" : "g";
+
+                        this.filterError = null;
+                        let regex;
+
+                        try {
+                            regex = new RegExp(text, flags);
+                        } catch (err) {
+                            regex = null;
+                            this.filterError = err.message;
+                        }
+
+                        if (regex)
+                            this.filterRegex = state.useWord ?
+                                new RegExp(`\\b(?:${text})\\b`, flags) :
+                                regex;
+                        else
+                            this.filterRegex = null;
                     }
 
                     this.updateUrl();