Python for All

Part One

What Is Regex?

Regex (regular expressions) is a way to search text using patterns instead of exact full text.

Think of it as a smart filter. Instead of saying "find exactly this one sentence", you say "find anything that looks like a phone number" or "find all 4-digit years".

In Python, regex lives in the re module, so we start with import re.

Regex syntax, step by step

1. Write a pattern as a raw string, for example r"\d{3}-\d{3}-\d{4}".
2. Break it into pieces: \d{3} = exactly 3 digits, - = a real dash, \d{3} = 3 digits, - = dash, \d{4} = 4 digits.
3. Read the whole pattern in plain English: "three digits, dash, three digits, dash, four digits."
4. Match that pattern against text to find values like 210-123-4567.

Part Two

Pattern Pieces (Slow Build)

Start with small pieces and combine them:

\d a digit, \w a letter/number/underscore, \s a space, . any single character.

Quantifiers: + one or more, * zero or more, ? optional, {n} exactly n, {n,m} between n and m times.

• \d: one digit. Example pattern r"\d" matches "7" in "Room 7".
• \w: letter/number/underscore. Example pattern r"\w+" matches words like "Athens2026".
• \s: whitespace. Example pattern r"\s" matches the space in "Hello World".
• .: any single character. Example pattern r"h.t" matches "hat", "hit", "hot".
• +: one or more. Example pattern r"\d+" matches "305" as one block.
• *: zero or more. Example pattern r"ab*" matches "a", "ab", "abbb".
• ?: optional (zero or one). Example pattern r"colou?r" matches both "color" and "colour".
• {n}: exactly n times. Example pattern r"\d{4}" matches years like "2026".
• {n,m}: from n to m times. Example pattern r"\d{2,4}" matches "12", "305", "2026".

Greek vs English letters (upper/lower)

When you need language-specific matching, use explicit character ranges:

• English uppercase: r"[A-Z]+"
• English lowercase: r"[a-z]+"
• English any case: r"[A-Za-z]+"
• Greek uppercase: r"[Α-ΩΆΈΉΊΌΎΏΪΫ]+"
• Greek lowercase: r"[α-ωάέήίόύώϊϋΐΰς]+"
• Greek any case: r"[Α-ΩΆΈΉΊΌΎΏΪΫα-ωάέήίόύώϊϋΐΰς]+"

Part Three

Regex Methods in re

Now that patterns make sense, use them with methods:

re.search() first match, re.findall() all matches, re.sub() replace matches.

• re.search(pattern, text) returns a Match object for the first match, or None if nothing matches.
• re.findall(pattern, text) returns a list of all matched strings (or an empty list if no matches).
• re.sub(pattern, replacement, text) returns a new string with replacements applied.
• re.fullmatch(pattern, text) returns a Match object only if the entire text matches, otherwise None.

What is a Match object? It is a result returned by regex when a match is found. Think of it as a small box with details about the match.

If you print the match object directly, Python shows something like: <re.Match object; span=(10, 14), match='2024'>. This is useful for debugging, but usually you want specific values from it.

Most common methods: match.group() (matched text), match.start() (start index), match.end() (end index).

What are start and end indexes? They are positions in the text. If the text is "Athens 2024, Patras", then the match "2024" starts at index 7 and ends at index 11. The end index is exclusive, so text[7:11] gives exactly "2024".

Part Four

Capturing Groups

Parentheses () create groups. Groups let you extract specific pieces of a match.

Group 1 is the first (), group 2 is the second, and so on.

Part Five

Pattern Pieces (More Advanced)

Now we add a few powerful pieces that you will use often in real text cleaning and validation.

^ start of string, $ end of string, | OR, [^abc] one character that is NOT in the set (a, b, c), \b word boundary, *? and +? lazy matching, \. literal dot.

• ^: start of text. Example pattern r"^A" matches names that start with A, like "Alex".
• $: end of text. Example pattern r"ing$" matches words ending in ing, like "learning".
• |: OR between options. Example pattern r"cat|dog" matches either "cat" or "dog".
• [^...]: one character not in a set. Example pattern r"[^0-9]+" matches non-numeric parts in "Room 205B" like "Room " and "B".
• \b: whole-word boundary. Example pattern r"\\bcat\\b" matches "cat" but not the cat inside "scatter".
• *? / +?: lazy matching (shortest possible). Example pattern r"<.*?>" matches each tag in "<b>bold</b><i>it</i>" one by one.
• \.: escaped dot for a real period. Example pattern r"python\\.org" matches "python.org" exactly.