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content/blog/capturing-quoted-string-sed.md

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+---
+title: "Capturing Quoted Strings in Sed"
+date: 2022-12-18T12:55:32-05:00
+draft: false
+tags: []
+math: false
+---
+
+*Disclaimer: This posts assumes some knowledge about regular expressions.*
+
+Recently I was trying to capture an HTML attribute in `sed`. For example, let's say I want to extract the `href` attribute in the following example:
+
+```
+<a href="https://brandonrozek.com" rel="me"></a>
+```
+
+Advice you commonly see on the Internet is to use a capture group for anything between the quotes of the href.
+
+In regular expression land, we can represent anything as `.*` and define a capture group of some regular expression `X` as `\(X\)`.
+
+```bash
+sed "s/.*href=\"\(.*\)\".*/\1/g"
+```
+
+What does this look like for our input?
+
+```bash
+echo \<a href=\"https://brandonrozek.com\" rel=\"me\"\>\</a\> |\
+sed "s/.*href=\"\(.*\)\".*/\1/g"
+```
+
+```
+https://brandonrozek.com" rel="me
+```
+
+It matches all the way until the second `"`! What we want, is to not match *any* character within the quotations, but match any character that is not the quotation itself `[^\"]*`
+
+```bash
+sed "s/.*href=\"\([^\"]*\)\".*/\1/g"
+```
+
+This then works for our example:
+
+```bash
+echo \<a href=\"https://brandonrozek.com\" rel=\"me\"\>\</a\> |\
+sed "s/.*href=\"\([^\"]*\)\".*/\1/g"
+```
+
+```
+https://brandonrozek.com
+```
+
+Within a bash script, we can make this a little more readable by using multiple variables.
+
+```bash
+QUOTED_STR="\"\([^\"]*\)\""
+BEFORE_TEXT=".*href=$QUOTED_STR.*"
+AFTER_TEXT="\1"
+REPLACE_EXPR="s/$BEFORE_TEXT/$AFTER_TEXT/g"
+
+INPUT="\<a href=\"https://brandonrozek.com\" rel=\"me\"\>\</a\>"
+
+echo "$INPUT" | sed "$REPLACE_EXPR"
+```
+