Regular expressions as a concept is not exclusive to Python at all.
- How To Use Regex In Python
- Regex Cheat Sheet
- C# Regex Cheat Sheet
- Regular Expression Cheat Sheet Python
How to download ccleaner for mac. Python, however, does have some nuances when it come to working with regular expressions.
This article is part of a series of articles on Python Regular Expressions.
In the first article of this series, we will focus on discussing how we work with regular expressions in python, highlighting python specifics.
We are going to introduce the methods we can use to perform queries over strings in Python. We’ll then talk about how we can use grouping to work with sub-parts of the matches we found using our queries.
The package we are interested in using to work with regular expressions in python is appropriately named ‘re’.
- Regular Expressions Cheat Sheet for Python, PHP, Perl, JavaScript and Ruby developers. The list of the most important metacharacters you'll ever need.
- Welcome to Python Cheatsheet! Anyone can forget how to make character classes for a regex, slice a list or do a for loop.This cheat sheet tries to provide a basic reference for beginner and advanced developers, lower the entry barrier for newcomers and help veterans refresh the old tricks.
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- Python Regular Expression's Cheat Sheet (borrowed from pythex) Special Characters. Escape special characters. Matches any character. ^ matches beginning of string. $ matches end of string. 5b-d matches any chars '5', 'b', 'c' or 'd'. ^a-c6 matches any char except 'a'.
1. Raw Strings in Python
The next column, 'Legend', explains what the element means (or encodes) in the regex syntax.
The python parser interprets ‘’ (backslashes) as escape characters in string literals.
If the backslash is followed by a special sequence recognized by the parser, the whole escape sequence is replaced by a corresponding special character (for example, ‘n’ is replaced by a newline character when processed by the parser).
This behavior causes a problem when working with regular expressions in python because the ‘re’ package also uses backslash characters to escape special regex characters (like * or +).
The combination of these two behaviors would mean that sometimes you would have to escape escape characters themselves (when the special character was recognized by both the python parser and the regex parser), yet other times you would not (if the special character was not recognized by the python parser).
Rather than bend our brains trying to figure out how many backslashes we need, we can instead use raw strings.
A raw string is created by simply adding an ‘r’ character before the opening quote of a normal string. When a string is raw, the python parser will not even attempt to make any substitutions within it. Essentially, you are telling the parser to completely leave your string alone.
Performing Queries with Regex in Python
The ‘re’ package provides several methods to actually perform queries on an input string. The methods that we will be discussing are:
- re.match()
- re.search()
- re.findall()
Each of the methods accepts a regular expression, and string to scan for matches. Lets take a look at each of these methods in a little more detail to see how they work and how they differ.
2. Find Using re.match – Matches Beginning
Lets first take a look at the match() method. The way the match() method works is that it will only find matches if they occur at the start of the string being searched.
So for example, calling match() on the string ‘dog cat dog’, looking for the pattern ‘dog’ will match:
We’ll be talking more about the group() method in a little bit. For now, just know that when called with 0 as it’s argument, the group() method returns the pattern matched by the query.
I’m also glossing over the returned SRE_Match object for now, we’ll talk about that in a minute too.
But, if we call match() on the same string, looking for the pattern ‘cat’, we won’t:
3. Find Using re.search – Matches Anywhere
The search() method is similar to match(), but search() doesn’t restrict us to only finding matches at the beginning of the string, so searching for ‘cat’ in our example string finds a match:
The search() method, however, stops looking after it finds a match, so search()-ing for ‘dog’ in our example string only finds the first occurrence:
4. Get Using re.findall – All Matching Objects
The querying method that I use by far the most in python though is the findall() method. Rather than being returned match objects (we’ll talk more about match objects in a little bit), when we call findall(), we simply get a list of all matching patterns. For me, this is just simpler. Calling findall() on our example string we get:
5. Use match.start and match.end Methods
So, what exactly are these ‘match objects’ that search() and match() gave us earlier?
Rather than simply return the matching portion of the string, search() and match() return ‘matches’, which are essentially a wrapper around the matching substring.
You saw earlier that I could get to the matching substring by calling the matches group() method, (match objects are actually pretty useful when it comes to working with grouping, as we will see in the next section), but the match object also contains much more information about the matching substring.
For example, the match object can tell us the start and end indexes of the matching content from the original string:
Knowing information like this can sometimes be very useful.
6. Group by Number Using match.group
How To Use Regex In Python
As I mentioned earlier, match objects come in very handy when working with grouping.
Grouping is the ability to address certain sub-parts of the entire regex match. We can define a group as a piece of the regular expression search string, and then individually address the corresponding content that was matched by this piece.
Let’s look at an example to see how this works:
The string I just created resembles a snippet taken out of someones address book. We can match the line with a regular expression like this one:
By surrounding certain parts of the regular expression in parentheses (the ‘(‘ and ‘)’ characters), we can group the content and then work with these individual groups.
These groups can be fetched using the match object’s group() method. The groups are addressable numerically in the order that they appear, from left to right, in the regular expression (starting with group 1):
The reason that the group numbering starts with group 1 is because group 0 is reserved to hold the entire match (we saw this earlier when we were learning about the match() and search() methods) Hootsuite for mac free download.
7. Grouping by Name Using match.group
Sometimes, especially when a regular expression has a lot of groups, it is impractical to address each group by its number. Python also allows you to assign a name to a group using the following syntax:
Hulu for mac download. When can still fetch the grouped content using the group() method, but this time specifying the names we assigned the groups instead of the numbering we used before:
This makes for much more explicit and readable code. You can imagine that as the regular expression became more and more complicated, understanding what was being captured by a grouping would get harder and harder. Assigning names to your groups explicitly informs you and your readers of your intentions.
Grouping can be used with the findall() method too, even though it doesn’t return match objects. Instead, findall() will return a list of tuples, where the Nth element of each tuple corresponds to the Nth group of the regex pattern:
However, named grouping doesn’t work when using the findall() method.
In this article we’ve introduced the basics of working with regular expressions in Python. We’ve learned about raw strings (and the headaches that they can save you when working with regular expressions). We’ve also learned how to perform basic queries using the match(), search(), and findall() methods, and even how to work with sub-components of a match using grouping.
As always, to find out more about this topic, the Python Official documentation on re package is a great resource.
In future articles, we’ll dive deeper into regular expressions in Python. We’ll talk about how we can capture an even broader range of matches, how we can use them to make substitutions within a string, and how we can even use them to parse python data structures out of text files.
The tables below are a reference to basic regex. While reading the rest of the site, when in doubt, you can always come back and look here. (It you want a bookmark, here's a direct link to the regex reference tables). I encourage you to print the tables so you have a cheat sheet on your desk for quick reference.The tables are not exhaustive, for two reasons. First, every regex flavor is different, and I didn't want to crowd the page with overly exotic syntax. For a full reference to the particular regex flavors you'll be using, it's always best to go straight to the source. In fact, for some regex engines (such as Perl, PCRE, Java and .NET) you may want to check once a year, as their creators often introduce new features.
The other reason the tables are not exhaustive is that I wanted them to serve as a quick introduction to regex. If you are a complete beginner, you should get a firm grasp of basic regex syntax just by reading the examples in the tables. I tried to introduce features in a logical order and to keep out oddities that I've never seen in actual use, such as the 'bell character'. With these tables as a jumping board, you will be able to advance to mastery by exploring the other pages on the site.
How to use the tables
The tables are meant to serve as an accelerated regex course, and they are meant to be read slowly, one line at a time. On each line, in the leftmost column, you will find a new element of regex syntax. The next column, 'Legend', explains what the element means (or encodes) in the regex syntax. The next two columns work hand in hand: the 'Example' column gives a valid regular expression that uses the element, and the 'Sample Match' column presents a text string that could be matched by the regular expression.You can read the tables online, of course, but if you suffer from even the mildest case of online-ADD (attention deficit disorder), like most of us… Well then, I highly recommend you print them out. You'll be able to study them slowly, and to use them as a cheat sheet later, when you are reading the rest of the site or experimenting with your own regular expressions.
Enjoy!
If you overdose, make sure not to miss the next page, which comes back down to Earth and talks about some really cool stuff: The 1001 ways to use Regex.
Regex Accelerated Course and Cheat Sheet
For easy navigation, here are some jumping points to various sections of the page:✽ Characters
✽ Quantifiers
✽ More Characters
✽ Logic
✽ More White-Space
✽ More Quantifiers
✽ Character Classes
✽ Anchors and Boundaries
✽ POSIX Classes
✽ Inline Modifiers
✽ Lookarounds
✽ Character Class Operations
✽ Other Syntax
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Characters
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| d | Most engines: one digit from 0 to 9 | file_dd | file_25 |
| d | .NET, Python 3: one Unicode digit in any script | file_dd | file_9੩ |
| w | Most engines: 'word character': ASCII letter, digit or underscore | w-www | A-b_1 |
| w | .Python 3: 'word character': Unicode letter, ideogram, digit, or underscore | w-www | 字-ま_۳ |
| w | .NET: 'word character': Unicode letter, ideogram, digit, or connector | w-www | 字-ま‿۳ |
| s | Most engines: 'whitespace character': space, tab, newline, carriage return, vertical tab | asbsc | a b c |
| s | .NET, Python 3, JavaScript: 'whitespace character': any Unicode separator | asbsc | a b c |
| D | One character that is not a digit as defined by your engine's d | DDD | ABC |
| W | One character that is not a word character as defined by your engine's w | WWWWW | *-+=) |
| S | One character that is not a whitespace character as defined by your engine's s | SSSS | Yoyo |
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Quantifiers
| Quantifier | Legend | Example | Sample Match |
|---|---|---|---|
| + | One or more | Version w-w+ | Version A-b1_1 |
| {3} | Exactly three times | D{3} | ABC |
| {2,4} | Two to four times | d{2,4} | 156 |
| {3,} | Three or more times | w{3,} | regex_tutorial |
| * | Zero or more times | A*B*C* | AAACC |
| ? | Once or none | plurals? | plural |
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More Characters
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| . | Any character except line break | a.c | abc |
| . | Any character except line break | .* | whatever, man. |
| . | A period (special character: needs to be escaped by a ) | a.c | a.c |
| Escapes a special character | .*+? $^/ | .*+? $^/ | |
| Escapes a special character | [{()}] | [{()}] |
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Logic
| Logic | Legend | Example | Sample Match |
|---|---|---|---|
| | | Alternation / OR operand | 22|33 | 33 |
| ( … ) | Capturing group | A(nt|pple) | Apple (captures 'pple') |
| 1 | Contents of Group 1 | r(w)g1x | regex |
| 2 | Contents of Group 2 | (dd)+(dd)=2+1 | 12+65=65+12 |
| (?: … ) | Non-capturing group | A(?:nt|pple) | Apple |
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More White-Space
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| t | Tab | Ttw{2} | T ab |
| r | Carriage return character | see below | |
| n | Line feed character | see below | |
| rn | Line separator on Windows | ABrnCD | AB CD |
| N | Perl, PCRE (C, PHP, R…): one character that is not a line break | N+ | ABC |
| h | Perl, PCRE (C, PHP, R…), Java: one horizontal whitespace character: tab or Unicode space separator | ||
| H | One character that is not a horizontal whitespace | ||
| v | .NET, JavaScript, Python, Ruby: vertical tab | ||
| v | Perl, PCRE (C, PHP, R…), Java: one vertical whitespace character: line feed, carriage return, vertical tab, form feed, paragraph or line separator | ||
| V | Perl, PCRE (C, PHP, R…), Java: any character that is not a vertical whitespace | ||
| R | Perl, PCRE (C, PHP, R…), Java: one line break (carriage return + line feed pair, and all the characters matched by v) |
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More Quantifiers
| Quantifier | Legend | Example | Sample Match |
|---|---|---|---|
| + | The + (one or more) is 'greedy' | d+ | 12345 |
| ? | Makes quantifiers 'lazy' | d+? | 1 in 12345 |
| * | The * (zero or more) is 'greedy' | A* | AAA |
| ? | Makes quantifiers 'lazy' | A*? | empty in AAA |
| {2,4} | Two to four times, 'greedy' | w{2,4} | abcd |
| ? | Makes quantifiers 'lazy' | w{2,4}? | ab in abcd |
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Character Classes
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| [ … ] | One of the characters in the brackets | [AEIOU] | One uppercase vowel |
| [ … ] | One of the characters in the brackets | T[ao]p | Tap or Top |
| - | Range indicator | [a-z] | One lowercase letter |
| [x-y] | One of the characters in the range from x to y | [A-Z]+ | GREAT |
| [ … ] | One of the characters in the brackets | [AB1-5w-z] | One of either: A,B,1,2,3,4,5,w,x,y,z |
| [x-y] | One of the characters in the range from x to y | [ -~]+ | Characters in the printable section of the ASCII table. |
| [^x] | One character that is not x | [^a-z]{3} | A1! |
| [^x-y] | One of the characters not in the range from x to y | [^ -~]+ | Characters that are not in the printable section of the ASCII table. |
| [dD] | One character that is a digit or a non-digit | [dD]+ | Any characters, inc- luding new lines, which the regular dot doesn't match |
| [x41] | Matches the character at hexadecimal position 41 in the ASCII table, i.e. A | [x41-x45]{3} | ABE |
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Anchors and Boundaries
| Anchor | Legend | Example | Sample Match |
|---|---|---|---|
| ^ | Start of string or start of line depending on multiline mode. (But when [^inside brackets], it means 'not') | ^abc .* | abc (line start) |
| $ | End of string or end of line depending on multiline mode. Many engine-dependent subtleties. | .*? the end$ | this is the end |
| A | Beginning of string (all major engines except JS) | Aabc[dD]* | abc (string.. ..start) |
| z | Very end of the string Not available in Python and JS | the endz | this is..n..the end |
| Z | End of string or (except Python) before final line break Not available in JS | the endZ | this is..n..the endn |
| G | Beginning of String or End of Previous Match .NET, Java, PCRE (C, PHP, R…), Perl, Ruby | ||
| b | Word boundary Most engines: position where one side only is an ASCII letter, digit or underscore | Bob.*bcatb | Bob ate the cat |
| b | Word boundary .NET, Java, Python 3, Ruby: position where one side only is a Unicode letter, digit or underscore | Bob.*bкошкаb | Bob ate the кошка |
| B | Not a word boundary | c.*BcatB.* | copycats |
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POSIX Classes
Regex Cheat Sheet
| Character | Legend | Example | Sample Match |
|---|---|---|---|
| [:alpha:] | PCRE (C, PHP, R…): ASCII letters A-Z and a-z | [8[:alpha:]]+ | WellDone88 |
| [:alpha:] | Ruby 2: Unicode letter or ideogram | [[:alpha:]d]+ | кошка99 |
| [:alnum:] | PCRE (C, PHP, R…): ASCII digits and letters A-Z and a-z | [[:alnum:]]{10} | ABCDE12345 |
| [:alnum:] | Ruby 2: Unicode digit, letter or ideogram | [[:alnum:]]{10} | кошка90210 |
| [:punct:] | PCRE (C, PHP, R…): ASCII punctuation mark | [[:punct:]]+ | ?!.,:; |
| [:punct:] | Ruby: Unicode punctuation mark | [[:punct:]]+ | ‽,:〽⁆ |
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C# Regex Cheat Sheet
Inline Modifiers
None of these are supported in JavaScript. In Ruby, beware of (?s) and (?m).| Modifier | Legend | Example | Sample Match |
|---|---|---|---|
| (?i) | Case-insensitive mode (except JavaScript) | (?i)Monday | monDAY |
| (?s) | DOTALL mode (except JS and Ruby). The dot (.) matches new line characters (rn). Also known as 'single-line mode' because the dot treats the entire input as a single line | (?s)From A.*to Z | From A to Z |
| (?m) | Multiline mode (except Ruby and JS) ^ and $ match at the beginning and end of every line | (?m)1rn^2$rn^3$ | 1 2 3 |
| (?m) | In Ruby: the same as (?s) in other engines, i.e. DOTALL mode, i.e. dot matches line breaks | (?m)From A.*to Z | From A to Z |
| (?x) | Free-Spacing Mode mode (except JavaScript). Also known as comment mode or whitespace mode | (?x) # this is a # comment abc # write on multiple # lines [ ]d # spaces must be # in brackets | abc d |
| (?n) | .NET, PCRE 10.30+: named capture only | Turns all (parentheses) into non-capture groups. To capture, use named groups. | |
| (?d) | Java: Unix linebreaks only | The dot and the ^ and $ anchors are only affected by n | |
| (?^) | PCRE 10.32+: unset modifiers | Unsets ismnx modifiers |
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Lookarounds
| Lookaround | Legend | Example | Sample Match |
|---|---|---|---|
| (?=…) | Positive lookahead | (?=d{10})d{5} | 01234 in 0123456789 |
| (?<=…) | Positive lookbehind | (?<=d)cat | cat in 1cat |
| (?!…) | Negative lookahead | (?!theatre)thew+ | theme |
| (?<!…) | Negative lookbehind | w{3}(?<!mon)ster | Munster |
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Character Class Operations
| Class Operation | Legend | Example | Sample Match |
|---|---|---|---|
| […-[…]] | .NET: character class subtraction. One character that is in those on the left, but not in the subtracted class. | [a-z-[aeiou]] | Any lowercase consonant |
| […-[…]] | .NET: character class subtraction. | [p{IsArabic}-[D]] | An Arabic character that is not a non-digit, i.e., an Arabic digit |
| […&&[…]] | Java, Ruby 2+: character class intersection. One character that is both in those on the left and in the && class. | [S&&[D]] | An non-whitespace character that is a non-digit. |
| […&&[…]] | Java, Ruby 2+: character class intersection. | [S&&[D]&&[^a-zA-Z]] | An non-whitespace character that a non-digit and not a letter. |
| […&&[^…]] | Java, Ruby 2+: character class subtraction is obtained by intersecting a class with a negated class | [a-z&&[^aeiou]] | An English lowercase letter that is not a vowel. |
| […&&[^…]] | Java, Ruby 2+: character class subtraction | [p{InArabic}&&[^p{L}p{N}]] | An Arabic character that is not a letter or a number |
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Other Syntax
Regular Expression Cheat Sheet Python
| Syntax | Legend | Example | Sample Match |
|---|---|---|---|
| Keep Out Perl, PCRE (C, PHP, R…), Python's alternate regex engine, Ruby 2+: drop everything that was matched so far from the overall match to be returned | prefixKd+ | 12 | |
| Perl, PCRE (C, PHP, R…), Java: treat anything between the delimiters as a literal string. Useful to escape metacharacters. | Q(C++ ?)E | (C++ ?) |
and The Best Regex Trick Ever!!!
The 1001 ways to use Regex
