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String Pattern Matching

Often used for data validation

Test against pattern (true/false)

Find matching groups
(and possibly replace content)

Regex Engines

Same language, engines.

For example: PHP native versus PCRE

POSIX (standard), BRE/ERE (Linux (grep)), Perl, java.util.regex, XRegExp (JS)

What do they look like?

/the regex goes in here/

`the regex goes in here`

"the regex goes in here"

"/the regex goes in here/"

Okay, but what does it really look like?

/foo/.test("foobar");

preg_match("/foo/", "foobar");

/foo/ =~ "foobar"

"foobar" =~ /foo/

Simple Character Test

/a/.test("JavaScript Rules");  // true

/z/.test("JavaScript Rules");  // false

Testing versus Matching

true/false vs. matched groups

/a/.test("JavaScript Rules");   // true
"JavaScript Rules".match(/a/);  // ["a"]

"JavaScript Rules".match(/z/);  // null

Where's the other "a"?

"JavaScript Rules".match(/a/);  // ["a"]

Other Flags

"JavaScript Rules".match(/r/);  // ["r"]

// global
"JavaScript Rules".match(/r/g);  // ["r"]

Multiple Characters

Any Character (.)

Classes, Ranges, and Repetition

[ ], ?, +, *, { }

Negation

[^]

Escaping Characters

\ . ? * + - | [ ] ( ) { } ^ $

Grouping and Alternation

( | )

Group Matching Caution

Creating a matched group adds processing time!

Replacement

This differs from engine to engine!

"I like {{fav}} the most".replace(/{{fav}}/, "dogs");

"I like dogs the most"

"jordan@jordankasper.com"
    .replace(/([a-z]+)@([a-z.]+)/i, "https://$2/users/$1");

"https://jordankasper.com/users/jordan"

Whitespace & Shorthands

Shorthand Example

Anchors

^ and $

Anchors

^ and $

Efficiency Warnings

Watch out for:

• Too many groups (use non-matching)
• Large source texts (use anchors)
• Unstructured data (like HTML)

HTML Warning

HTML is not well structured, be careful!

Resources

jordankasper.com/regex-101

• Playground: regex101.com
• Various Engine Info: Wikipedia
• Reference: regular-expressions.info
• Diagramer: regexper.com
• Dead Tree Version: "Mastering Regular Expressions"
• Fun: regexcrossword.com

Look Arounds

They are not matches, but assertions.

Positive Look Ahead

/(the)(?=\sfat)/i

The fat cat sat on the mat

Negative Look Ahead

/(the)(?!\sfat)/i

The fat cat sat on the mat

Look Behind

Same idea, but looking backward instead of forward.

Not supported in all languages!
(notabley: JavaScript)

Look Behind

/(?<=the\s)([a-z]at)/i

The fat cat sat on the mat

We also have negative look behind with: (?<!)