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+# Using find, matches, and sub for String Matching in WDL
+
+**When processing genomic data (or any data that includes string manipulations),** you may need to check whether your input strings follow a certain pattern, retrieve part of them, or replace certain substrings. With **WDL 1.2**, you can do this using three built-in functions—find, matches, and sub—which rely on POSIX Extended Regular Expressions (ERE).
+
+In this recipe, we’ll look at how to:
+- Detect if a pattern exists in your input strings (matches)
+- Extract the first match if it exists (find)
+- Replace occurrences of a regex pattern (sub)
+
+Just like any other WDL recipe, we’ll illustrate **why** you’d use these functions, **how** to integrate them into a minimal workflow, and **what** pitfalls to watch out for (like double-escaping backslashes).
+
+---
+
+## Problem
+
+You need a straightforward way to test or manipulate strings based on a regex pattern in your WDL workflow. Specifically, you want to:
+
+1. **Check** if a string contains some substring matching a certain pattern (returning true/false).
+2. **Find** the first substring that matches a certain pattern, returning that substring or None.
+3. **Replace** all occurrences of a certain pattern within a string.
+
+Before WDL 1.2, you had to rely on system calls or external scripts. Now, WDL 1.2 supplies built-in regex-based string functions for these operations.
+
+---
+
+## Solution
+
+WDL 1.2 offers **three** core functions in its standard library:
+
+1. **matches(String input, String pattern) → Boolean**  
+   Returns true if pattern occurs at least once in input, otherwise false.
+
+2. **find(String input, String pattern) → String?**  
+   Returns the *first* matching substring if pattern is found, or None if not found.
+
+3. **sub(String input, String pattern, String replace) → String**  
+   Replaces *all* non-overlapping occurrences of pattern in input with replace.
+
+**Important**:  
+- These functions rely on **POSIX Extended Regular Expressions**, so watch out for **double-escaping** backslashes.  
+- The WDL file must declare version 1.2 or higher, because matches and find are not available in older versions.
+
+---
+
+## Example: Minimal Workflow
+
+Below is a small example that demonstrates each function in a single workflow. The user provides an input_string and some patterns, and the workflow outputs the match result.
+
+wdl
+version 1.2
+
+workflow StringRegexExample {
+  input {
+    String input_string       # e.g. "Hello\tBob, sample_R1.fastq world!"
+    String pattern_for_find   # e.g. "\\t"
+    String pattern_for_match  # e.g. "_R1"
+    String pattern_for_sub    # e.g. "world"
+    String replace_string     # e.g. "WDL"
+  }
+
+  # Perform a find operation:
+  String? found_substring = find(input_string, pattern_for_find)
+
+  # Check if we have a match:
+  Boolean matched = matches(input_string, pattern_for_match)
+
+  # Replace any occurrence of pattern_for_sub:
+  String replaced_result = sub(input_string, pattern_for_sub, replace_string)
+
+  output {
+    String? found      = found_substring
+    Boolean is_matched = matched
+    String replaced    = replaced_result
+  }
+}
+
+
+### Sample Inputs
+
+json
+{
+  "StringRegexExample.input_string": "Hello\tBob, sample_R1.fastq world!",
+  "StringRegexExample.pattern_for_find": "\\t",
+  "StringRegexExample.pattern_for_match": "_R1",
+  "StringRegexExample.pattern_for_sub": "world",
+  "StringRegexExample.replace_string": "WDL"
+}
+
+
+### Running Locally with Cromwell
+
+bash
+java -jar cromwell.jar run StringRegexExample.wdl --inputs inputs.json
+
+
+**Possible Output**:
+
+- found: "\t" (the tab character), or None if not found  
+- is_matched: true (since _R1 is found in "sample_R1.fastq")  
+- replaced: "Hello\tBob, sample_R1.fastq WDL!"
+
+---
+
+## Key Points
+
+1. **Regex Anchors**: If you want to match the entire string, use ^ and $ (e.g., matches("abc123", "^a.+3$")).  
+2. **Double-Escaping**: For literal backslashes or special characters, remember to double-escape. "\\t" in WDL becomes a single \t in the regex engine.  
+3. **Conditional**: If you’re branching logic based on whether something matches, use if (matches(...)) { ... } or check if find(...) returns None.  
+4. **Return Types**:  
+   - matches(...) returns a Boolean  
+   - find(...) returns a String? (optional)  
+   - sub(...) returns a plain String.
+
+**That’s all there is to it!** By using WDL 1.2, you can easily incorporate string pattern matching and replacements into your workflows without resorting to external scripting.
+
+---
+
+## Additional Resources
+
+- [Official WDL 1.2 Specification](https://github.com/openwdl/wdl/blob/wdl-1.2/SPEC.md)  
+- [Regex Syntax: POSIX Extended Regular Expressions](https://en.wikipedia.org/wiki/Regular_expression#POSIX_extended)  
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