{ "cells": [ { "cell_type": "markdown", "id": "3dab4e57", "metadata": {}, "source": [ "# The `re` Module\n", "\n", "Python's built-in `re` module provides regex support. The 6 most commonly used regex functions are:\n", "\n", "| Function | Description | Sample Syntax | Return |\n", "|---|---|---|---|\n", "| `re.search()` | Find **first** match anywhere in the string | `re.search(pattern, text)` | **`Match` object** or `None` |\n", "| `re.match()` | Match only at the **start** of the string | `re.match(pattern, text)` | **`Match` object** or `None` |\n", "| `re.findall()` | Find **all** matches; return as a **list** | `re.findall(pattern, text)` | `list` of strings |\n", "| `re.sub()` | Find and replace matches | `re.sub(pattern, repl, text)` | `str` |\n", "| `re.split()` | Split string on a pattern | `re.split(pattern, text)` | `list` of strings |\n", "| `re.fulmatch()` | Match the entire string against the pattern | re.fullmatch(pattern, text) | **`Match` object** or `None` |" ] }, { "cell_type": "code", "execution_count": null, "id": "c7368273", "metadata": {}, "outputs": [], "source": [ "import sys\n", "from pathlib import Path\n", "\n", "# Find project root by looking for _config.yml\n", "current = Path.cwd()\n", "for parent in [current, *current.parents]:\n", " if (parent / '_config.yml').exists():\n", " project_root = parent\n", " break\n", "else:\n", " project_root = Path.cwd().parent.parent\n", "\n", "# Add project root to path\n", "sys.path.insert(0, str(project_root))\n", "\n", "# Import shared teaching helpers and cell magics\n", "from shared import thinkpython, diagram, jupyturtle, structshape\n", "from shared.download import download\n" ] }, { "cell_type": "markdown", "id": "d1950d45", "metadata": {}, "source": [ "## The `Match` object\n", "\n", "`re.search()`, `re.match()`, and `re.fullmatch()` functions return a `Match object` when pattern is matched. \n", "\n", "For example, \n", "- returns: `re.search(pattern, text)` scans through `text` and returns \n", " - a `Match` object for the **first** location where `pattern` is found. \n", " - If the pattern is not found anywhere in the string, it returns `None`. \n", "\n", "- `Match`: A `Match` object has the following commonly used attributes and methods: \n", "\n", "| Attribute / Method | Description | Example |\n", "|---|---|---|\n", "| `.group()` | Returns the matched substring | `m.group()` → `'Dracula'` |\n", "| `.start()` | Index where the match begins | `m.start()` → `5` |\n", "| `.end()` | Index where the match ends | `m.end()` → `12` |\n", "| `.span()` | Tuple of `(start, end)` | `m.span()` → `(5, 12)` |\n", "| `.string` | The original string that was searched | `m.string` → `'I am Dracula...'` |" ] }, { "cell_type": "code", "execution_count": 21, "id": "62713517", "metadata": {}, "outputs": [ { "data": { "text/plain": [ "" ] }, "execution_count": 21, "metadata": {}, "output_type": "execute_result" } ], "source": [ "import re\n", "\n", "text = \"I am Dracula; and I bid you welcome, Mr. Harker, to my house.\"\n", "pattern = 'Dracula'\n", "\n", "result = re.search(pattern, text) ### pattern: Dracula; text: the line\n", "result ### the Match object" ] }, { "cell_type": "markdown", "id": "ecf6690b", "metadata": {}, "source": [ "If the pattern appears in the text, `search` returns a `Match` object that contains the results of the search. \n", "\n", "1. String: Among other information, it has a variable named `string` that contains the text that was searched.\n", "2. Group: It also provides a method called `group` that returns the part of the text that **matched** the pattern.\n", "3. Span and Start/End: And it provides a method called `span` that returns the index in the text where the pattern starts and ends." ] }, { "cell_type": "code", "execution_count": 23, "id": "6fdd12a9", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "I am Dracula; and I bid you welcome, Mr. Harker, to my house.\n", "Dracula\n", "5\n", "12\n", "(5, 12)\n" ] } ], "source": [ "print(result.string)\n", "print(result.group())\n", "print(result.start())\n", "print(result.end())\n", "print(result.span())" ] }, { "cell_type": "markdown", "id": "d38ad650", "metadata": {}, "source": [ ":::{note}\n", "`.group()` returns the **matched substring from the text** — the portion of the text that the pattern matched against. In simple cases like `re.search('Dracula', text)`, the match equals the pattern string. But with a regex like `r'\\$[\\d.]+'`, `.group()` would return something like `'$42.99'` — the actual text that matched, not the pattern expression itself.\n", ":::" ] }, { "cell_type": "markdown", "id": "2fb89ac5", "metadata": {}, "source": [ "If the pattern doesn't appear in the text, the return value from `search` is `None`. So we can check whether the search was successful by checking whether the result is `None`." ] }, { "cell_type": "code", "execution_count": 18, "id": "613a304d", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "None\n" ] }, { "data": { "text/plain": [ "True" ] }, "execution_count": 18, "metadata": {}, "output_type": "execute_result" } ], "source": [ "result = re.search('Count', text)\n", "print(result)\n", "\n", "result is None" ] }, { "cell_type": "code", "execution_count": 5, "id": "3a3b88a7", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "['is', 'is']\n", "['is ', 'is ']\n", "['is ', 'is ']\n", "['is ', 'is ']\n", "['is ', 'is ']\n" ] } ], "source": [ "s = \"This is a test of the regular expression system.\"\n", "print(re.findall('is', s)) # ['is', 'is']\n", "print(re.findall('is.', s)) # ['is ', 'is '] ### 'is' followed by any character (space in this case)\n", "print(re.findall('is.?', s)) # ['is ', 'is '] ### 'is' followed by zero or one character (space in this case)\n", "print(re.findall('is.?', s, re.IGNORECASE)) # ['is ', 'is '] ### same as above, but case-insensitive \n", "print(re.findall('is.?', s, re.IGNORECASE | re.DOTALL)) # ['is ', 'is '] ### same as above, but also makes '.' match newline characters (not relevant in this case since there are no newlines)" ] }, { "cell_type": "markdown", "id": "ffef1c34", "metadata": {}, "source": [ "The `+` in the pattern means one or more occurrence. " ] }, { "cell_type": "code", "execution_count": 3, "id": "daa3fba6", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "['$42.99', '$7.50']\n", "$42.99\n", "13 19\n", "The price is PRICE and PRICE\n", "['one', 'two', 'three']\n" ] } ], "source": [ "import re\n", "\n", "text = \"The price is $42.99 and $7.50\"\n", "\n", "# findall — get all matches\n", "print(re.findall(r\"\\$[\\d.]+\", text)) # ['$42.99', '$7.50']\n", "\n", "# search — first match object\n", "m = re.search(r\"\\$[\\d.]+\", text)\n", "print(m.group()) # '$42.99'\n", "print(m.start(), m.end()) # position in string\n", "\n", "# sub — replace\n", "print(re.sub(r\"\\$[\\d.]+\", \"PRICE\", text)) # 'The price is PRICE and PRICE'\n", "\n", "# split\n", "print(re.split(r\"\\s+\", \"one two three\")) # ['one', 'two', 'three']" ] }, { "cell_type": "code", "execution_count": 50, "id": "96205219", "metadata": { "tags": [ "thebe-interactive" ] }, "outputs": [], "source": [ "### EXERCISE: Regex Escape Sequences\n", "# Difficulty: Basic\n", "s = \"Price: $19.95, code=A_7, spaces here\"\n", "# 1. Extract all digit sequences\n", "# 2. Extract all word tokens\n", "# 3. Extract literal '$' and literal '.' matches\n", "### Your code starts here:\n", "\n", "\n", "\n", "### Your code ends here." ] }, { "cell_type": "code", "execution_count": 51, "id": "8b965a41", "metadata": { "tags": [ "hide-input" ] }, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "['19', '95', '7']\n", "['Price', '19', '95', 'code', 'A_7', 'spaces', 'here']\n", "['$']\n", "['.']\n" ] } ], "source": [ "import re\n", "\n", "# Solution\n", "s = \"Price: $19.95, code=A_7, spaces here\"\n", "print(re.findall(r'\\d+', s))\n", "print(re.findall(r'\\w+', s))\n", "print(re.findall(r'\\$', s))\n", "print(re.findall(r'\\.', s))" ] }, { "cell_type": "code", "execution_count": null, "id": "8c5d4f62", "metadata": { "tags": [ "thebe-interactive" ] }, "outputs": [], "source": [ "### EXERCISE: The Match Object\n", "# Difficulty: Basic\n", "import re\n", "text = \"Customer ID: 4892, Order date: 2024-03-15\"\n", "# 1. Use re.search() to find the first 4-digit number in text\n", "# 2. Print the matched string, start index, end index, and span\n", "### Your code starts here:\n", "\n", "\n", "\n", "### Your code ends here." ] }, { "cell_type": "code", "execution_count": null, "id": "6408257c", "metadata": { "tags": [ "hide-input" ] }, "outputs": [], "source": [ "# Solution\n", "import re\n", "text = \"Customer ID: 4892, Order date: 2024-03-15\"\n", "m = re.search(r'\\d{4}', text)\n", "print(m.group())\n", "print(m.start())\n", "print(m.end())\n", "print(m.span())" ] } ], "metadata": { "kernelspec": { "display_name": ".venv", "language": "python", "name": "python3" }, "language_info": { "codemirror_mode": { "name": "ipython", "version": 3 }, "file_extension": ".py", "mimetype": "text/x-python", "name": "python", "nbconvert_exporter": "python", "pygments_lexer": "ipython3", "version": "3.13.7" } }, "nbformat": 4, "nbformat_minor": 5 }