Regular Expressions

Groovy has three regex operators on top of Java's java.util.regex:

def phone = /\d{3}-\d{4}/

println '555-1234' ==~ phone
println 'call 555-1234 now' ==~ phone
if ('call 555-1234 now' =~ phone) println 'found a phone number'
println((~/[aeiou]/).class.simpleName)

true
false
found a phone number
Pattern

Test your patterns interactively with the Regex Tester.

Extracting groups

def log = '2026-09-29 ERROR [db] connection lost'
def m = log =~ /(\d{4})-(\d\d)-(\d\d) (\w+) \[(\w+)\]/
if (m) {
    def (all, year, month, day, level, source) = m[0]
    println "$level from $source on $day/$month/$year"
}

def named = log =~ /(?<level>[A-Z]+) \[(?<src>\w+)\]/
named.find()
println named.group('level') + ' / ' + named.group('src')

ERROR from db on 29/09/2026
ERROR / db

m[0] is the first match as a list: the whole match followed by each group. Named groups ((?<name>...)) are read with group('name').

Finding all matches

def text = 'Order #123 shipped, #456 pending, #789 cancelled'

println text.findAll(/#\d+/)
println text.findAll(/#(\d+) (\w+)/) { all, id, status -> "$id:$status" }
println((text =~ /#\d+/).size())
text.eachMatch(/#(\d+)/) { all, id -> print "${id.toInteger() * 2} " }
println()

[#123, #456, #789]
[123:shipped, 456:pending, 789:cancelled]
3
246 912 1578

Replacing

def s = 'hello world from groovy'
println s.replaceAll(/o/, '0')
println s.replaceAll(/\b(\w)/) { all, first -> first.toUpperCase() }
println '2026-09-29'.replaceFirst(/(\d+)-(\d+)-(\d+)/, '$3.$2.$1')
println 'a1b22c333'.split(/\d+/)

hell0 w0rld fr0m gr00vy
Hello World From Groovy
29.09.2026
[a, b, c]

replaceAll with a closure computes each replacement; in a plain replacement string, $1 refers to a group.

Common mistakes

• Using =~ when you mean a full match: 'abc123' =~ /\d+/ is truthy because it finds digits. Use ==~ for validation.
• Writing regexes in double-quoted strings: "\d" is an invalid escape. Use slashy strings (/\d/).

Practice

Extract all email addresses from 'Contact ada@example.com or ops@test.org today' and print their domains.

def text = 'Contact ada@example.com or ops@test.org today'
def emails = text.findAll(/[\w.]+@[\w.]+\w/)
println emails
println emails.collect { it.split('@')[1] }

[ada@example.com, ops@test.org]
[example.com, test.org]