<html><head><link rel="stylesheet" href="resource://content-accessible/plaintext.css"><style media="screen" id="__markdown-viewer__md_css">/*
* Copyright (c) Sindre Sorhus <sindresorhus@gmail.com> (https://sindresorhus.com)
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
* this software and associated documentation files (the "Software"), to deal in
* the Software without restriction, including without limitation the rights to
* use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
* of the Software, and to permit persons to whom the Software is furnished to do
* so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
* SOFTWARE.
*/
@media (prefers-color-scheme: dark) {
:root {
--color-prettylights-syntax-comment: #8b949e;
--color-prettylights-syntax-constant: #79c0ff;
--color-prettylights-syntax-entity: #d2a8ff;
--color-prettylights-syntax-storage-modifier-import: #c9d1d9;
--color-prettylights-syntax-entity-tag: #7ee787;
--color-prettylights-syntax-keyword: #ff7b72;
--color-prettylights-syntax-string: #a5d6ff;
--color-prettylights-syntax-variable: #ffa657;
--color-prettylights-syntax-brackethighlighter-unmatched: #f85149;
--color-prettylights-syntax-invalid-illegal-text: #f0f6fc;
--color-prettylights-syntax-invalid-illegal-bg: #8e1519;
--color-prettylights-syntax-carriage-return-text: #f0f6fc;
--color-prettylights-syntax-carriage-return-bg: #b62324;
--color-prettylights-syntax-string-regexp: #7ee787;
--color-prettylights-syntax-markup-list: #f2cc60;
--color-prettylights-syntax-markup-heading: #1f6feb;
--color-prettylights-syntax-markup-italic: #c9d1d9;
--color-prettylights-syntax-markup-bold: #c9d1d9;
--color-prettylights-syntax-markup-deleted-text: #ffdcd7;
--color-prettylights-syntax-markup-deleted-bg: #67060c;
--color-prettylights-syntax-markup-inserted-text: #aff5b4;
--color-prettylights-syntax-markup-inserted-bg: #033a16;
--color-prettylights-syntax-markup-changed-text: #ffdfb6;
--color-prettylights-syntax-markup-changed-bg: #5a1e02;
--color-prettylights-syntax-markup-ignored-text: #c9d1d9;
--color-prettylights-syntax-markup-ignored-bg: #1158c7;
--color-prettylights-syntax-meta-diff-range: #d2a8ff;
--color-prettylights-syntax-brackethighlighter-angle: #8b949e;
--color-prettylights-syntax-sublimelinter-gutter-mark: #484f58;
--color-prettylights-syntax-constant-other-reference-link: #a5d6ff;
--color-fg-default: #c9d1d9;
--color-fg-muted: #8b949e;
--color-fg-subtle: #484f58;
--color-canvas-default: #0d1117;
--color-canvas-subtle: #161b22;
--color-border-default: #30363d;
--color-border-muted: #21262d;
--color-neutral-muted: rgba(110,118,129,0.4);
--color-accent-fg: #58a6ff;
--color-accent-emphasis: #1f6feb;
--color-danger-fg: #f85149;
}
}
@media (prefers-color-scheme: light) {
:root {
--color-prettylights-syntax-comment: #6e7781;
--color-prettylights-syntax-constant: #0550ae;
--color-prettylights-syntax-entity: #8250df;
--color-prettylights-syntax-storage-modifier-import: #24292f;
--color-prettylights-syntax-entity-tag: #116329;
--color-prettylights-syntax-keyword: #cf222e;
--color-prettylights-syntax-string: #0a3069;
--color-prettylights-syntax-variable: #953800;
--color-prettylights-syntax-brackethighlighter-unmatched: #82071e;
--color-prettylights-syntax-invalid-illegal-text: #f6f8fa;
--color-prettylights-syntax-invalid-illegal-bg: #82071e;
--color-prettylights-syntax-carriage-return-text: #f6f8fa;
--color-prettylights-syntax-carriage-return-bg: #cf222e;
--color-prettylights-syntax-string-regexp: #116329;
--color-prettylights-syntax-markup-list: #3b2300;
--color-prettylights-syntax-markup-heading: #0550ae;
--color-prettylights-syntax-markup-italic: #24292f;
--color-prettylights-syntax-markup-bold: #24292f;
--color-prettylights-syntax-markup-deleted-text: #82071e;
--color-prettylights-syntax-markup-deleted-bg: #FFEBE9;
--color-prettylights-syntax-markup-inserted-text: #116329;
--color-prettylights-syntax-markup-inserted-bg: #dafbe1;
--color-prettylights-syntax-markup-changed-text: #953800;
--color-prettylights-syntax-markup-changed-bg: #ffd8b5;
--color-prettylights-syntax-markup-ignored-text: #eaeef2;
--color-prettylights-syntax-markup-ignored-bg: #0550ae;
--color-prettylights-syntax-meta-diff-range: #8250df;
--color-prettylights-syntax-brackethighlighter-angle: #57606a;
--color-prettylights-syntax-sublimelinter-gutter-mark: #8c959f;
--color-prettylights-syntax-constant-other-reference-link: #0a3069;
--color-fg-default: #24292f;
--color-fg-muted: #57606a;
--color-fg-subtle: #6e7781;
--color-canvas-default: #ffffff;
--color-canvas-subtle: #f6f8fa;
--color-border-default: #d0d7de;
--color-border-muted: hsla(210,18%,87%,1);
--color-neutral-muted: rgba(175,184,193,0.2);
--color-accent-fg: #0969da;
--color-accent-emphasis: #0969da;
--color-danger-fg: #cf222e;
}
}
:root {
color: var(--color-fg-default);
background-color: var(--color-canvas-default);
}
.markdownroot {
-ms-text-size-adjust: 100%;
-webkit-text-size-adjust: 100%;
margin: 0;
font-family: -apple-system,BlinkMacSystemFont,"Segoe UI",Helvetica,Arial,sans-serif,"Apple Color Emoji","Segoe UI Emoji";
font-size: 16px;
line-height: 1.5;
word-wrap: break-word;
}
.markdownroot .octicon {
display: inline-block;
fill: currentColor;
vertical-align: text-bottom;
}
.markdownroot h1:hover .anchor .octicon-link:before,
.markdownroot h2:hover .anchor .octicon-link:before,
.markdownroot h3:hover .anchor .octicon-link:before,
.markdownroot h4:hover .anchor .octicon-link:before,
.markdownroot h5:hover .anchor .octicon-link:before,
.markdownroot h6:hover .anchor .octicon-link:before {
width: 16px;
height: 16px;
content: ' ';
display: inline-block;
background-color: currentColor;
-webkit-mask-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' version='1.1' aria-hidden='true'><path fill-rule='evenodd' d='M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z'></path></svg>");
mask-image: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' version='1.1' aria-hidden='true'><path fill-rule='evenodd' d='M7.775 3.275a.75.75 0 001.06 1.06l1.25-1.25a2 2 0 112.83 2.83l-2.5 2.5a2 2 0 01-2.83 0 .75.75 0 00-1.06 1.06 3.5 3.5 0 004.95 0l2.5-2.5a3.5 3.5 0 00-4.95-4.95l-1.25 1.25zm-4.69 9.64a2 2 0 010-2.83l2.5-2.5a2 2 0 012.83 0 .75.75 0 001.06-1.06 3.5 3.5 0 00-4.95 0l-2.5 2.5a3.5 3.5 0 004.95 4.95l1.25-1.25a.75.75 0 00-1.06-1.06l-1.25 1.25a2 2 0 01-2.83 0z'></path></svg>");
}
.markdownroot details,
.markdownroot figcaption,
.markdownroot figure {
display: block;
}
.markdownroot summary {
display: list-item;
}
.markdownroot a {
background-color: transparent;
color: var(--color-accent-fg);
text-decoration: none;
}
.markdownroot a:active,
.markdownroot a:hover {
outline-width: 0;
}
.markdownroot abbr[title] {
border-bottom: none;
-webkit-text-decoration: underline dotted;
text-decoration: underline dotted;
}
.markdownroot b,
.markdownroot strong {
font-weight: 600;
}
.markdownroot dfn {
font-style: italic;
}
.markdownroot h1 {
margin: .67em 0;
font-weight: 600;
padding-bottom: .3em;
font-size: 2em;
border-bottom: 1px solid var(--color-border-muted);
}
.markdownroot mark {
background-color: #ff0;
color: var(--color-text-primary);
}
.markdownroot small {
font-size: 90%;
}
.markdownroot sub,
.markdownroot sup {
font-size: 75%;
line-height: 0;
position: relative;
vertical-align: baseline;
}
.markdownroot sub {
bottom: -0.25em;
}
.markdownroot sup {
top: -0.5em;
}
.markdownroot img {
border-style: none;
max-width: 100%;
box-sizing: content-box;
background-color: var(--color-canvas-default);
}
.markdownroot code,
.markdownroot kbd,
.markdownroot pre,
.markdownroot samp {
font-family: monospace,monospace;
font-size: 1em;
}
.markdownroot figure {
margin: 1em 40px;
}
.markdownroot hr {
box-sizing: content-box;
overflow: hidden;
background: transparent;
border-bottom: 1px solid var(--color-border-muted);
height: .25em;
padding: 0;
margin: 24px 0;
background-color: var(--color-border-default);
border: 0;
}
.markdownroot html [type=button],
.markdownroot [type=reset],
.markdownroot [type=submit] {
-webkit-appearance: button;
}
.markdownroot [type=button]::-moz-focus-inner,
.markdownroot [type=reset]::-moz-focus-inner,
.markdownroot [type=submit]::-moz-focus-inner {
border-style: none;
padding: 0;
}
.markdownroot [type=button]:-moz-focusring,
.markdownroot [type=reset]:-moz-focusring,
.markdownroot [type=submit]:-moz-focusring {
outline: 1px dotted ButtonText;
}
.markdownroot [type=checkbox],
.markdownroot [type=radio] {
box-sizing: border-box;
padding: 0;
}
.markdownroot [type=number]::-webkit-inner-spin-button,
.markdownroot [type=number]::-webkit-outer-spin-button {
height: auto;
}
.markdownroot [type=search] {
-webkit-appearance: textfield;
outline-offset: -2px;
}
.markdownroot [type=search]::-webkit-search-cancel-button,
.markdownroot [type=search]::-webkit-search-decoration {
-webkit-appearance: none;
}
.markdownroot ::-webkit-input-placeholder {
color: inherit;
opacity: .54;
}
.markdownroot ::-webkit-file-upload-button {
-webkit-appearance: button;
font: inherit;
}
.markdownroot a:hover {
text-decoration: underline;
}
.markdownroot hr::before {
display: table;
content: "";
}
.markdownroot hr::after {
display: table;
clear: both;
content: "";
}
.markdownroot table {
border-spacing: 0;
border-collapse: collapse;
display: block;
width: max-content;
max-width: 100%;
overflow: auto;
}
.markdownroot td,
.markdownroot th {
padding: 0;
}
.markdownroot details summary {
cursor: pointer;
}
.markdownroot details:not([open])>*:not(summary) {
display: none !important;
}
.markdownroot kbd {
display: inline-block;
padding: 3px 5px;
font: 11px ui-monospace,SFMono-Regular,SF Mono,Menlo,Consolas,Liberation Mono,monospace;
line-height: 10px;
color: var(--color-fg-default);
vertical-align: middle;
background-color: var(--color-canvas-subtle);
border: solid 1px var(--color-neutral-muted);
border-bottom-color: var(--color-neutral-muted);
border-radius: 6px;
box-shadow: inset 0 -1px 0 var(--color-neutral-muted);
}
.markdownroot h1,
.markdownroot h2,
.markdownroot h3,
.markdownroot h4,
.markdownroot h5,
.markdownroot h6 {
margin-top: 24px;
margin-bottom: 16px;
font-weight: 600;
line-height: 1.25;
}
.markdownroot h2 {
font-weight: 600;
padding-bottom: .3em;
font-size: 1.5em;
border-bottom: 1px solid var(--color-border-muted);
}
.markdownroot h3 {
font-weight: 600;
font-size: 1.25em;
}
.markdownroot h4 {
font-weight: 600;
font-size: 1em;
}
.markdownroot h5 {
font-weight: 600;
font-size: .875em;
}
.markdownroot h6 {
font-weight: 600;
font-size: .85em;
color: var(--color-fg-muted);
}
.markdownroot p {
margin-top: 0;
margin-bottom: 10px;
}
.markdownroot blockquote {
margin: 0;
padding: 0 1em;
color: var(--color-fg-muted);
border-left: .25em solid var(--color-border-default);
}
.markdownroot ul,
.markdownroot ol {
margin-top: 0;
margin-bottom: 0;
padding-left: 2em;
}
.markdownroot ol ol,
.markdownroot ul ol {
list-style-type: lower-roman;
}
.markdownroot ul ul ol,
.markdownroot ul ol ol,
.markdownroot ol ul ol,
.markdownroot ol ol ol {
list-style-type: lower-alpha;
}
.markdownroot dd {
margin-left: 0;
}
.markdownroot tt,
.markdownroot code {
font-family: ui-monospace,SFMono-Regular,SF Mono,Menlo,Consolas,Liberation Mono,monospace;
font-size: 12px;
}
.markdownroot pre {
margin-top: 0;
margin-bottom: 0;
font-family: ui-monospace,SFMono-Regular,SF Mono,Menlo,Consolas,Liberation Mono,monospace;
font-size: 12px;
word-wrap: normal;
}
.markdownroot :-ms-input-placeholder {
color: var(--color-fg-subtle);
opacity: 1;
}
.markdownroot ::-ms-input-placeholder {
color: var(--color-fg-subtle);
opacity: 1;
}
.markdownroot ::placeholder {
color: var(--color-fg-subtle);
opacity: 1;
}
.markdownroot .pl-c {
color: var(--color-prettylights-syntax-comment);
}
.markdownroot .pl-c1,
.markdownroot .pl-s .pl-v {
color: var(--color-prettylights-syntax-constant);
}
.markdownroot .pl-e,
.markdownroot .pl-en {
color: var(--color-prettylights-syntax-entity);
}
.markdownroot .pl-smi,
.markdownroot .pl-s .pl-s1 {
color: var(--color-prettylights-syntax-storage-modifier-import);
}
.markdownroot .pl-ent {
color: var(--color-prettylights-syntax-entity-tag);
}
.markdownroot .pl-k {
color: var(--color-prettylights-syntax-keyword);
}
.markdownroot .pl-s,
.markdownroot .pl-pds,
.markdownroot .pl-s .pl-pse .pl-s1,
.markdownroot .pl-sr,
.markdownroot .pl-sr .pl-cce,
.markdownroot .pl-sr .pl-sre,
.markdownroot .pl-sr .pl-sra {
color: var(--color-prettylights-syntax-string);
}
.markdownroot .pl-v,
.markdownroot .pl-smw {
color: var(--color-prettylights-syntax-variable);
}
.markdownroot .pl-bu {
color: var(--color-prettylights-syntax-brackethighlighter-unmatched);
}
.markdownroot .pl-ii {
color: var(--color-prettylights-syntax-invalid-illegal-text);
background-color: var(--color-prettylights-syntax-invalid-illegal-bg);
}
.markdownroot .pl-c2 {
color: var(--color-prettylights-syntax-carriage-return-text);
background-color: var(--color-prettylights-syntax-carriage-return-bg);
}
.markdownroot .pl-sr .pl-cce {
font-weight: bold;
color: var(--color-prettylights-syntax-string-regexp);
}
.markdownroot .pl-ml {
color: var(--color-prettylights-syntax-markup-list);
}
.markdownroot .pl-mh,
.markdownroot .pl-mh .pl-en,
.markdownroot .pl-ms {
font-weight: bold;
color: var(--color-prettylights-syntax-markup-heading);
}
.markdownroot .pl-mi {
font-style: italic;
color: var(--color-prettylights-syntax-markup-italic);
}
.markdownroot .pl-mb {
font-weight: bold;
color: var(--color-prettylights-syntax-markup-bold);
}
.markdownroot .pl-md {
color: var(--color-prettylights-syntax-markup-deleted-text);
background-color: var(--color-prettylights-syntax-markup-deleted-bg);
}
.markdownroot .pl-mi1 {
color: var(--color-prettylights-syntax-markup-inserted-text);
background-color: var(--color-prettylights-syntax-markup-inserted-bg);
}
.markdownroot .pl-mc {
color: var(--color-prettylights-syntax-markup-changed-text);
background-color: var(--color-prettylights-syntax-markup-changed-bg);
}
.markdownroot .pl-mi2 {
color: var(--color-prettylights-syntax-markup-ignored-text);
background-color: var(--color-prettylights-syntax-markup-ignored-bg);
}
.markdownroot .pl-mdr {
font-weight: bold;
color: var(--color-prettylights-syntax-meta-diff-range);
}
.markdownroot .pl-ba {
color: var(--color-prettylights-syntax-brackethighlighter-angle);
}
.markdownroot .pl-sg {
color: var(--color-prettylights-syntax-sublimelinter-gutter-mark);
}
.markdownroot .pl-corl {
text-decoration: underline;
color: var(--color-prettylights-syntax-constant-other-reference-link);
}
.markdownroot [data-catalyst] {
display: block;
}
.markdownroot g-emoji {
font-family: "Apple Color Emoji","Segoe UI Emoji","Segoe UI Symbol";
font-size: 1em;
font-style: normal !important;
font-weight: 400;
line-height: 1;
vertical-align: -0.075em;
}
.markdownroot g-emoji img {
width: 1em;
height: 1em;
}
.markdownroot::before {
display: table;
content: "";
}
.markdownroot::after {
display: table;
clear: both;
content: "";
}
.markdownroot>*:first-child {
margin-top: 0 !important;
}
.markdownroot>*:last-child {
margin-bottom: 0 !important;
}
.markdownroot a:not([href]) {
color: inherit;
text-decoration: none;
}
.markdownroot .absent {
color: var(--color-danger-fg);
}
.markdownroot .anchor {
float: left;
padding-right: 4px;
margin-left: -20px;
line-height: 1;
}
.markdownroot .anchor:focus {
outline: none;
}
.markdownroot p,
.markdownroot blockquote,
.markdownroot ul,
.markdownroot ol,
.markdownroot dl,
.markdownroot table,
.markdownroot pre,
.markdownroot details {
margin-top: 0;
margin-bottom: 16px;
}
.markdownroot blockquote>:first-child {
margin-top: 0;
}
.markdownroot blockquote>:last-child {
margin-bottom: 0;
}
.markdownroot sup>a::before {
content: "[";
}
.markdownroot sup>a::after {
content: "]";
}
.markdownroot h1 .octicon-link,
.markdownroot h2 .octicon-link,
.markdownroot h3 .octicon-link,
.markdownroot h4 .octicon-link,
.markdownroot h5 .octicon-link,
.markdownroot h6 .octicon-link {
color: var(--color-fg-default);
vertical-align: middle;
visibility: hidden;
}
.markdownroot h1:hover .anchor,
.markdownroot h2:hover .anchor,
.markdownroot h3:hover .anchor,
.markdownroot h4:hover .anchor,
.markdownroot h5:hover .anchor,
.markdownroot h6:hover .anchor {
text-decoration: none;
}
.markdownroot h1:hover .anchor .octicon-link,
.markdownroot h2:hover .anchor .octicon-link,
.markdownroot h3:hover .anchor .octicon-link,
.markdownroot h4:hover .anchor .octicon-link,
.markdownroot h5:hover .anchor .octicon-link,
.markdownroot h6:hover .anchor .octicon-link {
visibility: visible;
}
.markdownroot h1 tt,
.markdownroot h1 code,
.markdownroot h2 tt,
.markdownroot h2 code,
.markdownroot h3 tt,
.markdownroot h3 code,
.markdownroot h4 tt,
.markdownroot h4 code,
.markdownroot h5 tt,
.markdownroot h5 code,
.markdownroot h6 tt,
.markdownroot h6 code {
padding: 0 .2em;
font-size: inherit;
}
.markdownroot ul.no-list,
.markdownroot ol.no-list {
padding: 0;
list-style-type: none;
}
.markdownroot ol[type="1"] {
list-style-type: decimal;
}
.markdownroot ol[type=a] {
list-style-type: lower-alpha;
}
.markdownroot ol[type=i] {
list-style-type: lower-roman;
}
.markdownroot div>ol:not([type]) {
list-style-type: decimal;
}
.markdownroot ul ul,
.markdownroot ul ol,
.markdownroot ol ol,
.markdownroot ol ul {
margin-top: 0;
margin-bottom: 0;
}
.markdownroot li>p {
margin-top: 16px;
}
.markdownroot li+li {
margin-top: .25em;
}
.markdownroot dl {
padding: 0;
}
.markdownroot dl dt {
padding: 0;
margin-top: 16px;
font-size: 1em;
font-style: italic;
font-weight: 600;
}
.markdownroot dl dd {
padding: 0 16px;
margin-bottom: 16px;
}
.markdownroot table th {
font-weight: 600;
}
.markdownroot table th,
.markdownroot table td {
padding: 6px 13px;
border: 1px solid var(--color-border-default);
}
.markdownroot table tr {
background-color: var(--color-canvas-default);
border-top: 1px solid var(--color-border-muted);
}
.markdownroot table tr:nth-child(2n) {
background-color: var(--color-canvas-subtle);
}
.markdownroot table img {
background-color: transparent;
}
.markdownroot img[align=right] {
padding-left: 20px;
}
.markdownroot img[align=left] {
padding-right: 20px;
}
.markdownroot .emoji {
max-width: none;
vertical-align: text-top;
background-color: transparent;
}
.markdownroot span.frame {
display: block;
overflow: hidden;
}
.markdownroot span.frame>span {
display: block;
float: left;
width: auto;
padding: 7px;
margin: 13px 0 0;
overflow: hidden;
border: 1px solid var(--color-border-default);
}
.markdownroot span.frame span img {
display: block;
float: left;
}
.markdownroot span.frame span span {
display: block;
padding: 5px 0 0;
clear: both;
color: var(--color-fg-default);
}
.markdownroot span.align-center {
display: block;
overflow: hidden;
clear: both;
}
.markdownroot span.align-center>span {
display: block;
margin: 13px auto 0;
overflow: hidden;
text-align: center;
}
.markdownroot span.align-center span img {
margin: 0 auto;
text-align: center;
}
.markdownroot span.align-right {
display: block;
overflow: hidden;
clear: both;
}
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</style><style id="__markdown-viewer__custom_css"></style><meta name="viewport" content="width=device-width, initial-scale=1"><title>Chapter 5: Using Regular Expressions in Production Rules</title></head><body><div id="__markdown-viewer__tools" class="floating"><input type="checkbox" id="__markdown-viewer__show-tools"><label for="__markdown-viewer__show-tools"></label><div id="__markdown-viewer__toc" class="toggleable"><ul><li><a href="#chapter-5-using-regular-expressions-in-production-rules">Chapter 5: Using Regular Expressions in Production Rules</a><ul><li><a href="#automatically-adding-new-rules-with-and">Automatically Adding New Rules with *, + and ?</a><ul><li><a href="#special-operators">Special Operators</a></li><li><a href="#nesting-restriction">Nesting Restriction</a></li></ul></li><li><a href="#json-parser">JSON Parser</a></li></ul></li></ul></div><p class="toggleable">Pick a markdown and code style:<br><select id="__markdown-viewer__mdselect"><option 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style="display: none;" href="blob:null/671ea1ff-1a82-42a6-9c44-9dc51b45dad0">Download as HTML</a></p></div><div class="markdownRoot"><h2 id="chapter-5-using-regular-expressions-in-production-rules">Chapter 5: Using Regular Expressions in Production Rules</h2>
<p>This chapter closely follows <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/chapter4.html">Chapter 4</a> as it is closely related
to how Rustlr generates ASTs.</p>
<h3 id="automatically-adding-new-rules-with-and">Automatically Adding New Rules with *, + and ?</h3>
<p>Rustlr allows the use of regular-expression style symbols *, + and ?,
which lead to the generate of new production rules and semantic actions
that create vectors and options (for ?). However, these
symbols cannot be used unrestrictedly to form arbitrary regular
expressions. They cannot be nested (see reason below).
They are also guaranteed to only fully work in the -auto mode.</p>
<p>Referring to the auto-ast generation example in <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/chapter4.html">Chapter 4</a>,
another way to define the nonterminal <code>ExprList</code> as a semicolon-separated
sequence of <code>LetExpr</code> is to replace the two productions for <code>ExprList</code> with
the following rule</p>
<pre><code><span class="hljs-selector-tag">ExprList</span> <span class="hljs-selector-tag">--</span>> (LetExpr ;)*
</code></pre>
<p>This would lead to the generation of a tuple struct for type ExprList:</p>
<pre><code><span class="hljs-meta">#[derive(Default,Debug)]</span>
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">ExprList</span></span><<span class="hljs-symbol">'lt</span>>(<span class="hljs-keyword">pub</span> <span class="hljs-built_in">Vec</span><LBox<Expr<<span class="hljs-symbol">'lt</span>>>>,);
</code></pre>
<p>The operator <strong><code>*</code></strong> means a sequence of zero or more. This is done by generating several new non-terminal symbols internally.
Essentially, these correspond to</p>
<pre><code>ES0 --> Expr:e ; {e}
ES1 --> { Vec::<span class="hljs-built_in">new</span>() }
ES1 --> ES1:v ES0:[e] { v.<span class="hljs-built_in">push</span>(e); v }
ExprList --> ES1:v {v}
</code></pre>
<p>These rules replace the original in the grammar. In the -auto mode,
rustlr also infers that symbols such as ; has no meaning at the
AST level (because it has the unit type and noprecedence/associativity
declaration). It therefore infers that the type of the nonterminal ES0
is the same as Expr, and automatically generates the appropriate semantic action.
If override of this behavior is required, one can manually rewrite the grammar
as</p>
<pre><code>ES0:SEMI <span class="hljs-comment">--> Expr ; </span>
ExprList <span class="hljs-comment">--> ES0*</span>
</code></pre>
<p>The presence of the left-hand side label will cause the AST generator to
create an AST representation for the semicolon (assuming that is what's
desired). Another situation where the user has to write the ES0 rule
manually is if <code>-auto</code> (or <code>-genabsyn</code>) is not specified, which implies
that a rule with an explicit semantic action is required. Generally speaking,
the *, + and ? symbols will still work without <code>-auto</code> if it follows a
single grammar symbol.</p>
<p>The type rustlr associates with the new non-terminal ES1
will be <code>Vec<LBox<Expr<'lt>></code> and semantic actions are generated to
create the vector for both ES1 rules. A <strong><code>+</code></strong> means one or more
<code>ES1</code> derivations, producing the same vector type, and a <strong><code>?</code></strong> will
mean one or zero derivations with type <code>Option<LBox<Expr<'lt>>></code>.</p>
<p>Another way to generate the AST for <code>ExprList</code> is to manually define the type
of <code>ExprList</code>, from which Rustlr will infer that it is a <code>pass-thru</code> case.
No type will be created for <code>ExprList</code> as it would inherit the type of
the right-hand side of its lone production rule.</p>
<pre><code>nonterminal ExprList <span class="hljs-built_in">Vec</span><LBox<Expr<<span class="hljs-symbol">'lt</span>>>>
ExprList --> (LetExpr ;)*
</code></pre>
<p>This is because rustlr generates an internal non-terminal to represent the right-hand side <code>*</code> expression and assigns it type <code>Vec<LBox<Expr<'lt>>></code>.
It then recognizes that this is the only symbol on the
right, which is of the same type as the left-hand side nonterminal <code>ExprList</code>
as declared. This rule will be given an action equivalent to
<code>ExprList --> (Expr ;)*:v {v}</code></p>
<p><strong>The label given for a regex-like expression cannot be a pattern that
includes <code>@...@</code>.</strong> It can only be a simple alphanumeric label or a
boxed label (<code>[x]</code>).</p>
<p>Another restriction is that the symbols <code>(</code>, <code>)</code>, <code>?</code>, <code>*</code> and <code>+</code> may not
be separated by white spaces since that would confuse their interpretation
as independent terminal symbols. For example, <code>( Expr ; ) *</code> is not valid.</p>
<h4 id="special-operators">Special Operators</h4>
<p>In addition to the <code>*</code>, <code>+</code> and <code>?</code> suffixes, rustlr also recognizes (non-nested)
suffixes such as <strong><code><Comma*></code></strong> or <strong><code><;+></code></strong>. Assuming that
<code>Comma</code> is a declared terminal symbol of the grammer, the expression
<code>Expr<Comma+></code> represents a sequence of one or more Expr separated by Comma,
but not ending in Comma, e.g <em>a,b,c</em> but not <em>a,b,c,</em>. In contrast,
<code>(Expr Comma)+</code> means that the expression must end in a Comma. <Comma*>
allows the sequence to be empty. The AST generator will also create vectors
as the semantic values of such expressions. Please avoid whitespaces in
these expressions: <code><Comma *></code> is not recognized.</p>
<h4 id="nesting-restriction">Nesting Restriction</h4>
<p>The regex operators <strong>cannot be nested.</strong> It is unlikely that
Rustlr will ever allow their nesting for such combinations can easily
be ambiguous. For example, with <strong><code>(a?)+</code></strong>, even a single <code>a</code>
will have an infinite number of derivations: as one <code>a?</code> or as three,
for example.</p>
<p>In general, be warned that overusing these regex-like operators,
especially in the same production, can easily lead to new
non-determinisms in the grammar. The new productions generated for
these operators could lead to additional Shift-Reduce and even
Reduce-Reduce conflicts. For example, a production with right-hand
side <strong><code>Expr<Comma*> Comma?</code></strong> will lead to a shift-reduce conflict.
However, <strong><code>Expr<Comma+> Comma?</code></strong> is fine and represents a
comma-separated sequence with an optional trailing comma.</p>
<p>Rustlr does its best to try to prevent the regex operators from causing
ambiguity. For example, it caches the uses of the operators to avoid
duplicate rules that are almost certain to cause conflicts.
However, mixing regular expressions and context-free grammars will still
have unexpected consequences for the user. We never have to worry about
a regex being "ambiguous" because they all reduce to a normal form (a
deterministic finite automaton with minimal states). The same is not
true for grammars. Grammars cannot be <em>combined</em> like regex can: for
example, a rule like <code>A --> B* B*</code> is hopelessly ambiguous as
there is no way to determine where the first sequence of B's should stop
and the second one begins.</p>
<p>The motivation for adding regex-like operators to a parser generator
are both usability and improved ability in creating abstract syntax
automatically. But much work needs to be done before we can parse
arbitrary EBNF syntax. We are currently exploring extensions of LR
parsing including <em>delayed reductions</em>, which can potentially allow
non-ambiguous grammars to be more easily composed without running into
new conflicts: see the <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/appendix.html">Appendix</a> of this tutorial for experimental
features.</p>
<hr>
<h3 id="json-parser">JSON Parser</h3>
<p>To give another, complete example of the features described in Chapters
4 and 5, we build a parser for JSON. The grammar is as follows</p>
<pre><code># Rustlr Grammar <span class="hljs-keyword">for</span> JSON
auto
lifetime <span class="hljs-symbol">'lt</span>
lexterminal LBRACE {
lexterminal RBRACE }
lexterminal LBRACK [
lexterminal RBRACK ]
lexterminal LPAREN (
lexterminal RPAREN )
lexterminal COLON :
lexterminal COMMA ,
lexterminal NULL null
lexterminal MINUS -
valueterminal TRUE~ <span class="hljs-built_in">bool</span>~ Alphanum(<span class="hljs-string">"true"</span>)~ <span class="hljs-literal">true</span>
valueterminal FALSE~ <span class="hljs-built_in">bool</span>~ Alphanum(<span class="hljs-string">"false"</span>)~ <span class="hljs-literal">false</span>
valueterminal STRING~ &<span class="hljs-symbol">'lt</span> <span class="hljs-built_in">str</span>~ Strlit(n)~ &n[<span class="hljs-number">1</span>..n.len()-<span class="hljs-number">1</span>]
valueterminal NUM~ <span class="hljs-built_in">i64</span>~ Num(n)~ n
valueterminal FLOAT~ <span class="hljs-built_in">f64</span>~ Float(n)~ n
valueterminal BIGNUM~ &<span class="hljs-symbol">'lt</span> <span class="hljs-built_in">str</span>~ BigNumber(n)~ n
nonterminal Integer <span class="hljs-built_in">i64</span>
nonterminal Floatpt <span class="hljs-built_in">f64</span>
nonterminal Boolean <span class="hljs-built_in">bool</span>
nonterminals Value KeyValuePair Number
nonterminal Object : Value
nonterminal List : Value
nonterminal Boolean : Value
topsym Value
resync COMMA RBRACK RBRACE
Integer --> MINUS?:m NUM:n {<span class="hljs-keyword">if</span> m.is_some() {n*-<span class="hljs-number">1</span>} <span class="hljs-keyword">else</span> {n}}
Floatpt --> MINUS?:m FLOAT:n {<span class="hljs-keyword">if</span> m.is_some() {-<span class="hljs-number">1.0</span>*n} <span class="hljs-keyword">else</span> {n}}
Number:Bignum --> MINUS?:m BIGNUM
Number:Int --> Integer
Number:Float --> Floatpt
Boolean --> TRUE | FALSE
Value:Number --> Number
Value:Boolean --> Boolean
Value:Str --> STRING
Value --> Object
Value --> List
Value --> NULL
Value --> LPAREN Value RPAREN
KeyValuePair --> STRING COLON Value
List:List --> LBRACK Value<COMMA*> RBRACK
Object:Object --> LBRACE KeyValuePair<COMMA*> RBRACE
</code></pre>
<p>This grammar uses the latest variations on Rustlr syntax that were introduced
in more recent versions.
Note that the <strong><code>valueterminal</code></strong> lines, which combine <code>typedterminal</code> with
<code>lexvalue</code> declarations, requires that their components be separated by
a <strong><code>~</code></strong>. The line for terminal STRING strips the string literal returned
by the tokenizer of this enclosing double quotes.</p>
<p>The grammar is a hybrid with some manually defined types and actions
along with automatically generated ones.
The AST types that are created by this grammar are</p>
<pre><code><span class="hljs-meta">#[derive(Debug)]</span>
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Number</span></span><<span class="hljs-symbol">'lt</span>> {
Int(<span class="hljs-built_in">i64</span>),
Bignum(<span class="hljs-built_in">Option</span><()>,&<span class="hljs-symbol">'lt</span> <span class="hljs-built_in">str</span>),
Float(<span class="hljs-built_in">f64</span>),
Number_Nothing,
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'lt</span>> <span class="hljs-built_in">Default</span> <span class="hljs-keyword">for</span> Number<<span class="hljs-symbol">'lt</span>> { <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">default</span></span>()-><span class="hljs-keyword">Self</span> { Number::Number_Nothing } }
<span class="hljs-meta">#[derive(Debug)]</span>
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Value</span></span><<span class="hljs-symbol">'lt</span>> {
Str(&<span class="hljs-symbol">'lt</span> <span class="hljs-built_in">str</span>),
Number(Number<<span class="hljs-symbol">'lt</span>>),
NULL,
Boolean(<span class="hljs-built_in">bool</span>),
Object(<span class="hljs-built_in">Vec</span><LBox<KeyValuePair<<span class="hljs-symbol">'lt</span>>>>),
List(<span class="hljs-built_in">Vec</span><LBox<Value<<span class="hljs-symbol">'lt</span>>>>),
Value_Nothing,
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'lt</span>> <span class="hljs-built_in">Default</span> <span class="hljs-keyword">for</span> Value<<span class="hljs-symbol">'lt</span>> { <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">default</span></span>()-><span class="hljs-keyword">Self</span> { Value::Value_Nothing } }
<span class="hljs-meta">#[derive(Default,Debug)]</span>
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">KeyValuePair</span></span><<span class="hljs-symbol">'lt</span>>(<span class="hljs-keyword">pub</span> &<span class="hljs-symbol">'lt</span> <span class="hljs-built_in">str</span>,<span class="hljs-keyword">pub</span> LBox<Value<<span class="hljs-symbol">'lt</span>>>,);
</code></pre>
<p>The following is the Debug-output of a sample AST produced by the parser,
from which anyone familiar with JSON can surely discern the original source:</p>
<pre><code><span class="hljs-keyword">Object</span>([KeyValuePair(<span class="hljs-string">"firstName"</span>, Str(<span class="hljs-string">"John"</span>)), KeyValuePair(<span class="hljs-string">"lastName"</span>, Str(<span class="hljs-string">"Smith"</span>)), KeyValuePair(<span class="hljs-string">"isAlive"</span>, <span class="hljs-keyword">Boolean</span>(<span class="hljs-literal">true</span>)), KeyValuePair(<span class="hljs-string">"age"</span>, Number(<span class="hljs-keyword">Int</span>(<span class="hljs-number">27</span>))), KeyValuePair(<span class="hljs-string">"address"</span>, <span class="hljs-keyword">Object</span>([KeyValuePair(<span class="hljs-string">"streetAddress"</span>, Str(<span class="hljs-string">"21 2nd Street"</span>)), KeyValuePair(<span class="hljs-string">"city"</span>, Str(<span class="hljs-string">"New York"</span>)), KeyValuePair(<span class="hljs-string">"state"</span>, Str(<span class="hljs-string">"NY"</span>)), KeyValuePair(<span class="hljs-string">"postalCode"</span>, Str(<span class="hljs-string">"10021-3100"</span>))])), KeyValuePair(<span class="hljs-string">"phoneNumbers"</span>, <span class="hljs-keyword">List</span>([<span class="hljs-keyword">Object</span>([KeyValuePair(<span class="hljs-string">"type"</span>, Str(<span class="hljs-string">"home"</span>)), KeyValuePair(<span class="hljs-string">"number"</span>, Str(<span class="hljs-string">"212 555-1234"</span>))]), <span class="hljs-keyword">Object</span>([KeyValuePair(<span class="hljs-string">"type"</span>, Str(<span class="hljs-string">"office"</span>)), KeyValuePair(<span class="hljs-string">"number"</span>, Str(<span class="hljs-string">"646 555-4567"</span>))])])), KeyValuePair(<span class="hljs-string">"children"</span>, <span class="hljs-keyword">List</span>([Str(<span class="hljs-string">"Catherine"</span>), Str(<span class="hljs-string">"Thomas"</span>), Str(<span class="hljs-string">"Trevor"</span>)])), KeyValuePair(<span class="hljs-string">"spouse"</span>, <span class="hljs-literal">NULL</span>)])
</code></pre>
<p>Finally, here is an alternative way to parse JSON objects into
Rust HashMaps. We can override not only the type to be generated but the
semantic action as well. Modify the declarations and rules for Object as
follows:</p>
<pre><code>$<span class="hljs-keyword">use</span> std::collections::HashMap;
nonterminal Object HashMap<&<span class="hljs-symbol">'lt</span> <span class="hljs-built_in">str</span>, LBox<@Value>>
Object ==> LBRACE KeyValPair<COMMA*>:entries RBRACE {
<span class="hljs-keyword">let</span> <span class="hljs-keyword">mut</span> kvmap = HashMap::new();
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">mut</span> lbx) <span class="hljs-keyword">in</span> entries {
<span class="hljs-keyword">if</span> <span class="hljs-keyword">let</span> KeyValPair(k,v) = lbx.take() { kvmap.insert(k,v); }
}
kvmap
} <==
</code></pre>
<p>The <code>$</code> directive is similar to <code>!</code>, except that it adds the verbatim line
only to the generated AST file as opposed to parser file. The syntax
<strong><code>@Value</code></strong> refers to the type of the <code>Value</code> nonterminal (or you can say
<code>Value<'lt></code>, but you will in general know what type would be generated
by the system for the nonterminal: the <code>@</code> symbol offers a convenience.)</p>
<p>Each key-value pair inside the vector created by <code><COMMA*></code> is wrapped
inside an LBox. We can take the value from the box with
<a href="https://docs.rs/rustlr/latest/rustlr/generic_absyn/struct.LBox.html#method.take">LBox::take</a> (which leaves a default value inside the box).</p>
<p>The debug output of the AST from the same JSON source would now be:</p>
<pre><code><span class="hljs-selector-tag">Object</span>({"spouse": NULL, <span class="hljs-string">"age"</span>: <span class="hljs-built_in">Number</span>(<span class="hljs-built_in">Int</span>(<span class="hljs-number">27</span>)), <span class="hljs-string">"phoneNumbers"</span>: <span class="hljs-built_in">List</span>([<span class="hljs-built_in">Object</span>({<span class="hljs-string">"type"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"home"</span>), <span class="hljs-string">"number"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"212 555-1234"</span>)}), <span class="hljs-selector-tag">Object</span>({"number": <span class="hljs-built_in">Str</span>(<span class="hljs-string">"646 555-4567"</span>), <span class="hljs-string">"type"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"office"</span>)})]), "children": <span class="hljs-built_in">List</span>([<span class="hljs-built_in">Str</span>(<span class="hljs-string">"Catherine"</span>), <span class="hljs-built_in">Str</span>(<span class="hljs-string">"Thomas"</span>), <span class="hljs-built_in">Str</span>(<span class="hljs-string">"Trevor"</span>)]), <span class="hljs-string">"lastName"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"Smith"</span>), <span class="hljs-string">"address"</span>: <span class="hljs-built_in">Object</span>({<span class="hljs-string">"streetAddress"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"21 2nd Street"</span>), <span class="hljs-string">"city"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"New York"</span>), <span class="hljs-string">"state"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"NY"</span>), <span class="hljs-string">"postalCode"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"10021-3100"</span>)}), "isAlive": <span class="hljs-built_in">Boolean</span>(true), <span class="hljs-string">"firstName"</span>: <span class="hljs-built_in">Str</span>(<span class="hljs-string">"John"</span>)})
</code></pre>
<p>Although we may at times want to insert such overrides, most of the
automatically generated portions remain usable.</p>
<p>Here are links to the <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/jsontypes/json.grammar">grammar</a>, <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/jsontypes/src/jsonparser.rs">parser</a>, <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/jsontypes/src/json_ast.rs">ast types</a> and <a href="https://cs.hofstra.edu/~cscccl/rustlr_project/jsontypes/src/main.rs">main</a> of the project, which may differ slightly
from the above.</p>
<hr>
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